[clangd] Incorporate transitive #includes into code complete proximity scoring.

Summary:
We now compute a distance from the main file to the symbol header, which
is a weighted count of:
 - some number of #include traversals from source file --> included file
 - some number of FS traversals from file --> parent directory
 - some number of FS traversals from parent directory --> child file/dir
This calculation is performed in the appropriate URI scheme.

This means we'll get some proximity boost from header files in main-file
contexts, even when these are in different directory trees.

This extended file proximity model is not yet incorporated in the index
interface/implementation.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D48441

llvm-svn: 336177