Implemented [[clang::musttail]] attribute for guaranteed tail calls.

This is a Clang-only change and depends on the existing "musttail"
support already implemented in LLVM.

The [[clang::musttail]] attribute goes on a return statement, not
a function definition. There are several constraints that the user
must follow when using [[clang::musttail]], and these constraints
are verified by Sema.

Tail calls are supported on regular function calls, calls through a
function pointer, member function calls, and even pointer to member.

Future work would be to throw a warning if a users tries to pass
a pointer or reference to a local variable through a musttail call.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D99517