Refine patch a bit.

- signed_ptr/SP -> ptrauth/CPA
- Verifier.cpp: check CPA type == base ptr type
- C API: exclude ConstantPtrAuth until needed
- BitcodeAnalyzer.cpp: print CST_CODE_PTRAUTH
- IR: refine test a bit, add addrspace for base/disc
- docs: refine a bit, remove stale overload
- docs: add to PointerAuth.md