[clangd] Add tests covering existing header-guard behavior. NFC

A few different mechanisms here that will need some work to untangle:
 - self-include in a preamble being an error even if the file is ifdef-guarded
 - the is-include-guarded flag not being propagated from preamble to main ast
 - preambles containing the first half on an include guard discard that info

For now just record current behavior.

Relevant to:
- https://github.com/clangd/clangd/issues/811
- https://github.com/clangd/clangd/issues/377
- https://github.com/clangd/clangd/issues/262

Differential Revision: https://reviews.llvm.org/D106201