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

This adds the `__swift_objc_members__` attribute to the semantic
analysis.  It allows for annotating ObjC interfaces to provide Swift
semantics indicating that the types derived from this interface will be
back-bridged to Objective-C to allow interoperability with Objective-C
and 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/D87395
Reviewed By: Aaron Ballman, Dmitri Gribenko