[clang][extract-api] Emit "functionSignature" in SGF for ObjC methods.

- Split GlobalRecord into two distinct types to be able to introduce
has_function_signature type trait.
- Add has_function_signature type trait.
- Serialize function signatures as part of serializeAPIRecord for
records that are known to have a function signature.

Differential Revision: https://reviews.llvm.org/D123304