Sema: introduce `__attribute__((__swift_name__))`

This introduces the new `swift_name` attribute that allows annotating
APIs with an alternate spelling for Swift.  This is used as part of the
importing mechanism to allow interfaces to be imported with a new name
into Swift.  It takes a parameter which is the Swift function name.
This parameter is validated to check if it matches the possible
transformed signature in Swift.

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/D87534
Reviewed By: Aaron Ballman, Dmitri Gribenko