Add a C++11 and C2x spelling for the type safety attribute...

Add a C++11 and C2x spelling for the type safety attribute (argument_with_type_tag, pointer_with_type_tag, and type_tag_for_datatype) in the clang vendor namespace.

The TypeTagForDatatype attribute had custom parsing rules that previously prevented it from being supported with square bracket notation. The ArgumentWithTypeTag attribute previously had unnecessary custom parsing that could be handled declaratively.

llvm-svn: 326052