Sema: add support for `__attribute__((__swift_bridged_typedef__))`

Extend the semantic attributes that clang processes for Swift to include
`swift_bridged_typedef`.  This attribute enables typedefs to be bridged
into Swift with a bridged name.

This is based on the work of the original changes in
https://github.com/llvm/llvm-project-staging/commit/8afaf3aad2af43cfedca7a24cd817848c4e95c0c

Differential Revision: https://reviews.llvm.org/D87396
Reviewed By: Aaron Ballman