[AArch64] - Return address signing dwarf support

Functions that have signed return addresses need additional dwarf support:
- After signing the LR, and before authenticating it, the LR register is in a
  state the is unusable by a debugger or unwinder
- To account for this a new directive, .cfi_negate_ra_state, is added
- This directive says the signed state of the LR register has now changed,
  i.e. unsigned -> signed or signed -> unsigned
- This directive has the same CFA code as the SPARC directive GNU_window_save
  (0x2d), adding a macro to account for multiply defined codes
- This patch matches the gcc implementation of this support:
  https://patchwork.ozlabs.org/patch/800271/

Differential Revision: https://reviews.llvm.org/D50136

llvm-svn: 343089