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

This attribute allows declarations to be restricted to the framework
itself, enabling Swift to remove the declarations when importing
libraries.  This is useful in the case that the functions can be
implemented in a more natural way for 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/D87720
Reviewed By: Aaron Ballman