Eliminate the last remaining header NDEBUG

VerifyDiagnosticConsumer is long-lived so the two additional members shouldn't
have any impact on release builds.

The clang headers are now free of NDEBUG conditionals. Let's keep it that way!

Note that they're not yet structurally stable, pending a few fixes in the LLVM
core headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196739 91177308-0d34-0410-b5e6-96231b3b80d8