[AST] Break down IdentTypeRepr to different subtypes.

This makes memory allocation for it more efficient and it's more convenient to handle.

Swift SVN r12541

lib/Parse/ParseDecl.cpp

@@ -919,7 +919,7 @@ ParserResult<ExtensionDecl> Parser::parseDeclExtension(unsigned Flags,
                                 diag::invalid_diagnostic).isError())
       return nullptr;
     Ty = makeParserErrorResult(
-        IdentTypeRepr::createSimple(Context, NameLoc, ExtensionName));
+        new (Context) SimpleIdentTypeRepr(NameLoc, ExtensionName));
   }
   if (Ty.isNull())
     return nullptr;