[Clang] Introduce Swift async calling convention.

This change is intended as initial setup. The plan is to add
more semantic checks later. I plan to update the documentation
as more semantic checks are added (instead of documenting the
details up front). Most of the code closely mirrors that for
the Swift calling convention. Three places are marked as
[FIXME: swiftasynccc]; those will be addressed once the
corresponding convention is introduced in LLVM.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D95561