CommentLexer: eliminate an NDEBUG from the headers

Code in headers shouldn't be conditional on the build configuration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196656 91177308-0d34-0410-b5e6-96231b3b80d8
Eliminate the last trivial NDEBUG uses in clang headers

assert(sanity()) reads so much better than preprocessor conditional blocks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196657 91177308-0d34-0410-b5e6-96231b3b80d8