[AArch64] Branch Protection and Return Address Signing B Key Support

- Add support for -mbranch-protection=<type>[+<type>]* where
  - <type> ::= [standard, none, bti, pac-ret[+b-key,+leaf]*]
- The protection emits relevant function attributes
  - sign-return-address=<scope>
  - sign-return-address-key=<key>
  - branch-protection

llvm-svn: 345273