[clang-tools-extra] [clangd] Add a unit test suite for HeuristicResolver (PR #121313)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

LGTM, feel free to merge unless you prefer additional input from other maintainers.

https://github.com/llvm/llvm-project/pull/121313