"Use" lambda captures which are otherwise only used in asserts. NFC

Summary:
The LLVM coding standards recommend "using" values that are only
needed by asserts:
http://llvm.org/docs/CodingStandards.html#assert-liberally

Without this change, LLVM cannot bootstrap with -Werror as the second
stage fails with this new warning:
https://reviews.llvm.org/rL291905

See also the previous fixes:
https://reviews.llvm.org/rL291916
https://reviews.llvm.org/rL291939
https://reviews.llvm.org/rL291940
https://reviews.llvm.org/rL291941

Reviewers: rsmith

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28695

llvm-svn: 291957