Add `LambdaCapture`-related matchers.

This provides better support for `LambdaCapture`s by making them first-
class and allowing them to be bindable. In addition, this implements several
`LambdaCapture`-related matchers. This does not update how lambdas are
traversed. As a result, something like trying to match `lambdaCapture()` by
itself will not work - it must be used as an inner matcher.

Reviewed By: aaron.ballman, sammccall

Differential Revision: https://reviews.llvm.org/D112491