[Sema] Fix crash in unused-lambda-capture warning for VLAs
Clang was crashing when diagnosing an unused-lambda-capture for a VLA because
From.getVariable() is null for the capture of a VLA bound.
Warning about the VLA bound capture is not helpful, so only warn for the VLA
Reviewers: aaron.ballman, dim, rsmith
Reviewed By: aaron.ballman, dim
Differential Revision: https://reviews.llvm.org/D41016