[clang][extract-api] Use relative includes

This patch transforms the given input headers to relative include names
using header search entries and some heuritics.
For example: `/Path/To/Header.h` will be included as `<Header.h>` with a
search path of `-I /Path/To/`; and
`/Path/To/Framework.framework/Headers/Header.h` will be included as
`<Framework/Header.h>`, given a search path of `-F /Path/To`.
Headermaps will also be queried in reverse to find a spelled name to
include headers.

Differential Revision: https://reviews.llvm.org/D123831