Change the Symbol Graph serializer for ExtractAPI to use `objective-c`
for the language name string for Objective-C, to align with clang
frontend standards.
- The name SymbolGraph is inappropriate and confusing for the new library
for clang-extract-api. Refactor and rename things to make it clear that
ExtractAPI is the core functionality and SymbolGraph is one serializer
for the API information.
- Add documentation comments to ExtractAPI classes and methods to improve
readability and clearness of the ExtractAPI work.
Differential Revision: https://reviews.llvm.org/D122160
