Fix spurious Wunused-lambda-capture warning

Summary:
Clang emits unused-lambda-capture warning for captures in generic lambdas even though they are actually used.

Fixes PR31815.

Reviewers: malcolm.parsons, aaron.ballman, rsmith

Reviewed By: malcolm.parsons

Subscribers: ahatanak, cfe-commits

Differential Revision: https://reviews.llvm.org/D33526

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305315 91177308-0d34-0410-b5e6-96231b3b80d8