Fix DumpIL error message for IL stubs (dotnet#5731)

IL stubs (P/Invoke and Reverse P/Invoke marshaling stubs) have a nil
metadata token (0x06000000) and no IL body. The DumpIL command now
detects this case early via IsNilToken and prints a clear message
instead of the cryptic 'error decoding IL' / 'ilAddr is 0'. Also
improved the fallback error message when GetILAddress fails for non-nil
tokens.

This is the right fix, as IL for stubs is freed after JIT'ing in
FreeCompileTimeState. Sometimes they persist if the IL was put on the
loader heap for a couple of specific stubs, but it's not worth building
a big feature to display these in SOS, so we'll improve the message and
be done.

Fixes dotnet#3005

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: leculver

src/SOS/Strike/strike.cpp

@@ -790,10 +790,16 @@ DECLARE_API(DumpIL)
         }
         else
         {
+            if (IsNilToken(MethodDescData.MDToken))
+            {
+                ExtOut("This method has no IL body (e.g. IL stubs generated for P/Invoke or Reverse P/Invoke marshaling).\n");
+                return S_OK;
+            }
+
             GetILAddressResult result = GetILAddress(MethodDescData);
             if (std::get<0>(result) == (TADDR)0)
             {
-                ExtOut("ilAddr is %p\n", SOS_PTR(std::get<0>(result)));
+                ExtOut("Unable to retrieve IL for this method (ilAddr is %p).\n", SOS_PTR(std::get<0>(result)));
                 return E_FAIL;
             }
             ExtOut("ilAddr is %p pImport is %p\n", SOS_PTR(std::get<0>(result)), SOS_PTR(std::get<1>(result)));