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

Add the `swift_newtype` attribute which allows a type definition to be
imported into Swift as a new type.  The imported type must be either an
enumerated type (enum) or an object type (struct).

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/D87652
Reviewed By: Aaron Ballman