[clangd] Extract mapper logic from clangd-indexer into a library.

Summary: Soon we can drop support for MR-via-YAML.
I need to modify some out-of-tree versions to use the library, first.

Reviewers: kadircet

Subscribers: mgorny, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52465

llvm-svn: 343019