[CodeGen/DWARF] Introduce DWARF tag for SwiftTail and emit it in CodeGen.

swifttailcc is a new calling convention in LLVM introduced in
https://reviews.llvm.org/D95443. We add a new DWARF tag to capture
this in debuginfo.

Differential Revision: https://reviews.llvm.org/D95704

[Note: This patch seems to have been forgotten about, landing it now with considerable delay]