[CodeGen] Fix codegen for __attribute__((swiftasynccall)).

[varungandhi-apple]

Summary

1. It should be mapped to LLVM's swifttailcc, which is now available.
2. We should make sure we generate a tail call instead of an ordinary call.

Fixes rdar://73762895.

[varungandhi-apple]

I am planning to add more tests, figured it is better to put up the patch sooner rather than later.

[varungandhi-apple]

Generate tail call when doing codegen for return statement.

[rjmccall]

Okay, seems fine to me.

clang/lib/CodeGen/CGStmt.cpp

1156

Hmm. I guess this should work in any situation where we can successfully actually do a tail call.

[rjmccall]

Please add some C++ tests, just in case.

[varungandhi-apple]

Added checks for C++.

[rjmccall]

LGTM

[varungandhi-apple]

Add null check for callee; instead of using getFunctionType() directly.

Otherwise, it leads to a crash when compiling gtest-all.cc.

[varungandhi-apple]

Use musttail call instead of tail call. (This patch needs to be marked as dependent on another patch which implements proper musttail support, but that is not available yet.)

[varungandhi-apple]

Fix codegen + add tests for methods and function pointers.

[rjmccall]

clang/lib/CodeGen/CGStmt.cpp

1240

What's with the explicit :: here?

[varungandhi-apple]

1. Remove '::' when calling static function.
2. Fix bug in function merging around missing musttail.
3. Add off-by-default check in verifier for swifttailcc->swifttailcc tail calls that are not marked musttail.