Clean up Binder.GetWellKnownType API (#45665)

src/Compilers/CSharp/Portable/Binder/Binder_Symbols.cs

@@ -1505,9 +1505,9 @@ internal NamedTypeSymbol GetWellKnownType(WellKnownType type, DiagnosticBag diag
         /// This is a layer on top of the Compilation version that generates a diagnostic if the well-known
         /// type isn't found.
         /// </summary>
-        internal NamedTypeSymbol GetWellKnownType(CSharpCompilation compilation, WellKnownType type, DiagnosticBag diagnostics, SyntaxNode node)
+        internal static NamedTypeSymbol GetWellKnownType(CSharpCompilation compilation, WellKnownType type, DiagnosticBag diagnostics, SyntaxNode node)
         {
-            return GetWellKnownType(this.Compilation, type, diagnostics, node.Location);
+            return GetWellKnownType(compilation, type, diagnostics, node.Location);
         }
 
         internal static NamedTypeSymbol GetWellKnownType(CSharpCompilation compilation, WellKnownType type, DiagnosticBag diagnostics, Location location)