[WebKit Checkers] Add alpha.webkit.UnborrowedLambdaCapturesChecker (#226288)
Like alpha.webkit.UnborrowedLocalVarsChecker, but for lambda captures.
This checker is pretty restrictive because, unlike a refcounted object,
a lifetime-dependent pointer/reference/view cannot be captured in an
escaping closure at all. Still, we permit capturing in a NOESCAPE
closure.