[clangd] IncludeCleaner as a library: Find all references to symbols in the file

This is the first patch in an ongoing attempt of Include Cleaner: unused/missing
headere diagnostics, an IWYU-like functionality implementation for clangd. The
work is split into (mostly) distinct and parallelizable pieces:

- Finding all referenced locations (this patch).
- Finding all referenced locations of macros.
- Building IncludeGraph and marking headers as unused, used and directly used.
- Making use of the introduced library and add an option to use in clangd.

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* Adding support for standard library headers (possibly through mapping
  genfiles).

Based on https://reviews.llvm.org/D100540.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D105426