Update vendored System.Runtime.CompilerServices.Unsafe (#8469)

## Summary of changes

- Create re-usable vendoring process for
System.Runtime.CompilerServices.Unsafe
- Update the vendored code (it's actually unchanged)

## Reason for change

We want to be able to update vendored code as required. For the vendored
[System.Runtime.CompilerServices.Unsafe
package](https://nuget.info/packages/System.Runtime.CompilerServices.Unsafe/6.1.2),
the [source
code](https://github.com/dotnet/maintenance-packages/blob/14e29655e53aec37342e933bfd7ba574167453ff/src/System.Runtime.CompilerServices.Unsafe/src/System.Runtime.CompilerServices.Unsafe.il)
is written directly as IL, and [compiled using
Ilasm.exe](https://learn.microsoft.com/en-us/dotnet/framework/tools/ilasm-exe-il-assembler).
So this PR introduces a simple "IL to C#" converter that converts that
file to its C# equivalent, using the same `InlineIL.Fody` approach that
we currently have.

The final result produces a file that is _virtually_ identical to the
existing one (which is good!). The only difference I made was to add the
original IL line as a comment next to the Fody equivalent.

This also shows that the code has not _actually_ changed (and it's
unlikely it _will_ tbh), so this just means we have a repeatable
self-contained approach to regenerate this in the repo as required.

## Implementation details

Told 🤖 to make the IL to C# converter, and it did 😄 I've given the code
it generated a once-over to look for anything terrible, but the key
thing is that the _output_ is sane, and that's visibly basically
unchanged, so I think it's fine.

## Test coverage

@dudikeleti already wrote some extensive tests for the `Unsafe`
implementation back when he originally ported it, which verifies that we
can compile the methods, call them, and that the generated IL is
identical to the "real" versions 🎉

## Other details


https://datadoghq.atlassian.net/browse/APMLP-1207

Part of a stack updating our vendored system code

- #8391
- #8454
- #8455
- #8459
- #8461

Author: andrewlock

tracer/build/_build/UpdateVendors/ILToInlineILConverter.cs

@@ -310,10 +310,31 @@ static VendoredDependency()
 
             Add(
                 libraryName: "System.Runtime.CompilerServices.Unsafe",
-                version: "1.0.0",
-                downloadUrl: "https://github.com/DataDog/dotnet-vendored-code/archive/refs/tags/1.0.0.zip",
-                pathToSrc: new[] { "dotnet-vendored-code-1.0.0", "System.Reflection.Metadata", "System.Runtime.CompilerServices.Unsafe" },
-                transform: filePath => RewriteCsFileWithStandardTransform(filePath, originalNamespace: "System.Runtime", AddNullableDirectiveTransform, AddIgnoreNullabilityWarningDisablePragma));
+                version: "6.1.2",
+                // Download link for commit 14e29655e53aec37342e933bfd7ba574167453ff from https://github.com/dotnet/maintenance-packages
+                downloadUrl: "https://codeload.github.com/dotnet/maintenance-packages/zip/14e29655e53aec37342e933bfd7ba574167453ff",
+                pathToSrc: new[] { "maintenance-packages-14e29655e53aec37342e933bfd7ba574167453ff", "src", "System.Runtime.CompilerServices.Unsafe", "src" },
+                transform: filePath =>
+                {
+                    // Convert .il file to InlineIL-based .cs file
+                    if (string.Equals(Path.GetFileName(filePath), "System.Runtime.CompilerServices.Unsafe.il", StringComparison.OrdinalIgnoreCase))
+                    {
+                        var ilContent = File.ReadAllText(filePath);
+                        var csContent = ILToInlineILConverter.Convert(ilContent);
+
+                        // Remove IL file, and replace with c# file instead
+                        File.Delete(filePath);
+                        filePath = Path.Combine(Path.GetDirectoryName(filePath)!, "Unsafe.cs");
+                        File.WriteAllText(
+                            filePath,
+                            csContent,
+                            new UTF8Encoding(encoderShouldEmitUTF8Identifier: false));
+
+                        // Use the full namespace to avoid rewriting BCL usings
+                        RewriteCsFileWithStandardTransform(filePath, originalNamespace: "System.Runtime.CompilerServices.Unsafe");
+                    }
+                },
+                onlyIncludePaths: new[] { "System.Runtime.CompilerServices.Unsafe.il", });
 
             Add(
                 libraryName: "ICSharpCode.SharpZipLib",

tracer/build/_build/UpdateVendors/VendoredDependency.cs

@@ -43,17 +43,14 @@
     <!-- https://www.nuget.org/packages/System.Memory/4.6.3 -->
     <PackageReference Include="System.Memory" Version="4.6.3" />
 
-    <!-- https://www.nuget.org/packages/System.Private.CoreLib/1.0.0 -->
-    <PackageReference Include="System.Private.CoreLib" Version="1.0.0" />
-
     <!-- https://www.nuget.org/packages/System.Reflection.Metadata.Interop/7.0.20 -->
     <PackageReference Include="System.Reflection.Metadata.Interop" Version="7.0.20" />
 
     <!-- https://www.nuget.org/packages/System.Reflection.Metadata/7.0.20 -->
     <PackageReference Include="System.Reflection.Metadata" Version="7.0.20" />
 
-    <!-- https://www.nuget.org/packages/System.Runtime.CompilerServices.Unsafe/1.0.0 -->
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="1.0.0" />
+    <!-- https://www.nuget.org/packages/System.Runtime.CompilerServices.Unsafe/6.1.2 -->
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.2" />
 
     <!-- https://www.nuget.org/packages/ICSharpCode.SharpZipLib/1.3.3 -->
     <PackageReference Include="ICSharpCode.SharpZipLib" Version="1.3.3" />