[Sema] Add warning for unused lambda captures

Summary:
Warn when a lambda explicitly captures something that is not used in its body.

The warning is part of -Wunused and can be enabled with -Wunused-lambda-capture.

Reviewers: rsmith, arphaman, jbcoe, aaron.ballman

Subscribers: Quuxplusone, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D28467

llvm-svn: 291905