[clang][ExtractAPI] Add support for single symbol SGF and libclang support

This is mainly adding an entry point to `SymbolGraphSerializer` at
`serializeSingleSymbolSGF` and exposing the necessary data to make this
possible.  Additionaly there are some changes to how symbol kinds and
path components are serialized to make the usage more ergonomic in
`serializeSingleSymbolSGF`.

On the libclang side this introduces APIs to:
- create an APISet from a TU
- dispose of an APISet
- query an APISet for a single symbol SGF for a given USR.
- generate a single symbol SGF for a given CXCursor, this only traverses
the necessary AST nodes to construct the result as oppposed as going
through the entire AST.

Differential Revision: https://reviews.llvm.org/D139115