[clang] Allow all string types for all attribute(format) styles

This allows using any recognized kind of string for any
__attribute__((format)) archetype. Before this change, for instance,
the printf archetype would only accept char pointer types and the
NSString archetype would only accept NSString pointers. This is
more restrictive than necessary as there exist functions to
convert between string types that can be annotated with
__attribute__((format_arg)) to transfer format information.

Reviewed By: ahatanak

Differential Revision: https://reviews.llvm.org/D125254

rdar://89060618