[llvm] [clang] New calling convention preserve_none (PR #76868)

[David Li via cfe-commits]

https://github.com/david-xl commented:

Can you increase the test coverage:

1) preserve_none caller TAIL-calls preserve_none callee -- the tail call is preserved
2) regular function TAIL-calls preserve_none callee -- the tail call should be disabled and all CSRs should be saved/restored around the call.
3) preserve-none caller calls preserve_none callee -- no registers are saved/restored around the call
4) preserve_none caller calls preserve_none callee with long argument list -- CSRs used for arg passing.

https://github.com/llvm/llvm-project/pull/76868