[Sema] Reword unused lambda capture warning

Summary:
The warning doesn't know why the variable was looked up but not
odr-used, so reword it to not claim that it was used in an unevaluated
context.

Reviewers: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28902

llvm-svn: 292498