AArch64: Cleanup tailcall CC check, enable swiftcc.

AArch64: Cleanup tailcall CC check, enable swiftcc.

Cleanup/change the code that checks for possible tailcall conventions to
look the same as the one in the X86 target. This makes the distinction
between calling conventions that can guarnatee tailcalls and the ones
that may tailcall more obvious.

- Add Swift to the mayTailCall list
- PreserveMost seemed to be incorrectly part of the guarnteed tail call
  list, move it to the mayTailCall list.

llvm-svn: 281376