Propagate the 'this' keyword in LibraryImports (#102793)

Since Roslyn just lowers the extension methods to a call to the static method there's no reason we shouldn't support creating extension methods that are also LibraryImport methods. This adds the IsExplicitThis property to TypePositionInfo and copies this to the generated signature if it is true.

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/MarshallerHelpers.cs

@@ -426,6 +426,11 @@ public static SyntaxTokenList GetManagedParameterModifiers(TypePositionInfo type
                 }
             }
 
+            if (typeInfo.IsExplicitThis)
+            {
+                tokens = tokens.Add(Token(SyntaxKind.ThisKeyword));
+            }
+
             return tokens;
         }
 

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/TypePositionInfo.cs

@@ -3,9 +3,10 @@
 
 using System;
 using System.Collections.Generic;
-
+using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
 using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
 
 namespace Microsoft.Interop
@@ -77,6 +78,7 @@ public static int IncrementIndex(int index)
 
         public int ManagedIndex { get; init; } = UnsetIndex;
         public int NativeIndex { get; init; } = UnsetIndex;
+        public bool IsExplicitThis { get; init; }
 
         public static TypePositionInfo CreateForParameter(IParameterSymbol paramSymbol, MarshallingInfo marshallingInfo, Compilation compilation)
         {
@@ -88,7 +90,8 @@ public static TypePositionInfo CreateForParameter(IParameterSymbol paramSymbol,
                 RefKind = paramSymbol.RefKind,
                 ByValueContentsMarshalKind = byValueContentsMarshalKind,
                 ByValueMarshalAttributeLocations = (inLocation, outLocation),
-                ScopedKind = paramSymbol.ScopedKind
+                ScopedKind = paramSymbol.ScopedKind,
+                IsExplicitThis = ((ParameterSyntax)paramSymbol.DeclaringSyntaxReferences[0].GetSyntax()).Modifiers.Any(SyntaxKind.ThisKeyword)
             };
 
             return typeInfo;