[libclang] Separate the underlying indexing functionality of libclang and introduce it into the clangIndex library.

It is a general goodness for libclang itself to mostly be a wrapper of functionality provided by the libraries.

llvm-svn: 260760
GitOrigin-RevId: f4fb85b1c74e88c7b3d4346e5a52348f97bb7ecd
18 files changed