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

This extends semantic analysis of attributes for Swift interoperability
by introducing the `swift_bridge` attribute.  This attribute enables
bridging Objective-C types to Swift specific types.

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/D87532
Reviewed By: Aaron Ballman