[DebugInfo] Teach LLVM and LLDB about ptrauth in DWARF

Teach libDebugInfo (llvm-dwarfdump) and lldb about DWARF tags and
attributes for pointer authentication. These values have been emitted by
Apple clang for several releases. Although upstream LLVM doesn't emit
these values yet, we hope to upstream that part sometime soon.

Differential revision: https://reviews.llvm.org/D130215