Show register names in DWARF unwind info.

Show register names in DWARF unwind info.

Register context information was already being passed into the DWARFDebugFrame code that dumps unwind information but it wasn't being used. This change adds the ability to dump registers names of a valid MC register context was passed in and if it knows about the register. Updated the tests to use the newly returned register names.

Differential Revision: https://reviews.llvm.org/D88767