Make String.ThrowSubstringArgumentOutOfRange static (#130103)

Makes `ThrowSubstringArgumentOutOfRange` static so it no longer takes
`this`. Passing the string's `Length` as an argument instead potentially
makes `this` more escape-analysis friendly since we don't pass it to
`Throw*` anymore.

> [!NOTE]
> This PR was generated with the assistance of GitHub Copilot.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -2292,7 +2292,7 @@ public string Substring(int startIndex)
 
             if ((uint)startIndex > (uint)Length)
             {
-                ThrowSubstringArgumentOutOfRange(startIndex, length);
+                ThrowSubstringArgumentOutOfRange(startIndex, length, Length);
             }
 
             return InternalSubString(startIndex, length);
@@ -2307,7 +2307,7 @@ public string Substring(int startIndex, int length)
             if ((uint)startIndex > (uint)Length || (uint)length > (uint)(Length - startIndex))
 #endif
             {
-                ThrowSubstringArgumentOutOfRange(startIndex, length);
+                ThrowSubstringArgumentOutOfRange(startIndex, length, Length);
             }
 
             if (length == 0)
@@ -2325,11 +2325,11 @@ public string Substring(int startIndex, int length)
         }
 
         [DoesNotReturn]
-        private void ThrowSubstringArgumentOutOfRange(int startIndex, int length)
+        private static void ThrowSubstringArgumentOutOfRange(int startIndex, int length, int thisLength)
         {
             ArgumentOutOfRangeException.ThrowIfNegative(startIndex);
 
-            if (startIndex > Length)
+            if (startIndex > thisLength)
             {
                 throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndexLargerThanLength);
             }