[clang] [alpha.webkit.UncheckedLambdaCapturesChecker] Split unchecked lambda capture checker (PR #201044)

[Ryosuke Niwa via cfe-commits]

rniwa wrote:

> I think, ideally we had 2 commits (or PRs). One that does the NFC split, and the focused other patch for improving the diags. Mixing the two is bad practice.

OK, I've split the PR into two commits. The first one improves diagnostics and the second one splits the checker.

https://github.com/llvm/llvm-project/pull/201044