[clang] [clang-tools-extra] [clangd] show lambda name instead of operator() in signature help (PR #101857)

[Timothy Akintilo via cfe-commits]

tilobyte wrote:

> I also wonder if we would better off having `CXXRecordDecl` save its 'variable' for lambdas (and look it up from there) rather than this.

saving the `VarDecl` in the lambda's `CXXRecordDecl` indeed sounds like a cleaner solution--i will try implementing that before making any changes to `AddMethodCandidate()`'s signature, since it wouldn't need the `LambdaDecl` parameter any more with this solution.

https://github.com/llvm/llvm-project/pull/101857