Extensions: allow extern and disallow in nameof

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8189,6 +8189,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>An extension member syntax is disallowed in nested position within an extension member syntax</value>
   </data>
   <data name="ERR_NameofExtensionMember" xml:space="preserve">
-    <value>Extension member are not allowed as an argument to 'nameof'.</value>
+    <value>Extension members are not allowed as an argument to 'nameof'.</value>
   </data>
 </root>

src/Compilers/CSharp/Portable/Emitter/Model/MethodSymbolAdapter.cs

@@ -438,7 +438,7 @@ Cci.IPlatformInvokeInformation Cci.IMethodDefinition.PlatformInvokeData
         System.Reflection.MethodImplAttributes Cci.IMethodDefinition.GetImplementationAttributes(EmitContext context)
         {
             CheckDefinitionInvariant();
-            return AdaptedMethodSymbol.ImplementationAttributes;
+            return AdaptedMethodSymbol.ContainingType.IsExtension ? default : AdaptedMethodSymbol.ImplementationAttributes;
         }
 
         bool Cci.IMethodDefinition.IsRuntimeSpecial