Fix "warning: fallthrough annotation does not directly precede switch label" in lambdas.

Summary: This patch fixes http://llvm.org/PR17864 - "warning: fallthrough annotation does not directly precede switch label" in lambdas.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D4258