[clang-tools-extra] [clang-tidy] Centralize the logic for matching discarded expressions (PR #184069)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Missing some discarded expressions:

- LHS of comma operator
- LHS of member expression (`A.B`/`A->B`) if RHS is enumerator or static member
- explicitly casted to `void`

https://github.com/llvm/llvm-project/pull/184069