[include-cleaner] Include-cleaner library structure, and simplistic AST walking.

Include-cleaner is a library that uses the clang AST and preprocessor to
determine which headers are used. It will be used in clang-tidy, in
clangd, in a standalone tool at least for testing, and in out-of-tree tools.

Roughly, it walks the AST, finds referenced decls, maps these to
used sourcelocations, then to FileEntrys, then matching these against #includes.
However there are many wrinkles: dealing with macros, standard library
symbols, umbrella headers, IWYU directives etc.

It is not built on the C++20 modules concept of usage, to allow:
 - use with existing non-modules codebases
 - a flexible API embeddable in clang-tidy, clangd, and other tools
 - avoiding a chicken-and-egg problem where include cleanups are needed
   before modules can be adopted

This library is based on existing functionality in clangd that provides
an unused-include warning. However it has design changes:
 - it accommodates diagnosing missing incl...