[clangd] Add tests that no-op changes are cheap

Summary:
We want to be sure they don't cause AST rebuilds or evict items from the cache.
D77847 is going to start sending spurious no-op changes (in case the preamble
was invalidated), this is cheap enough but we shouldn't regress that in future.

Reviewers: kadircet

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78048