[clangd] Basic IncludeCleaner support for c/c++ standard library

There are some limitations here, so this is behind a flag for now (in addition
to the config setting for the overall feature).

- symbols without exactly one associated header aren't handled right
- no macro support
- referencing std::size_t usually doesn't leave any trace in the AST that the
  alias in std was used, so we associate with stddef.h instead of cstddef.
  (An AST issue not specific to stdlib, but much worse there)

Differential Revision: https://reviews.llvm.org/D114077