CommentLexer: eliminate an NDEBUG from the headers

Code in headers shouldn't be conditional on the build configuration.
Eliminate the last trivial NDEBUG uses in clang headers

assert(sanity()) reads so much better than preprocessor conditional blocks.
CodeGen: Don't emit linkage on thunks that aren't emitted because they're vararg.

This can happen when we're trying to emit a thunk with available_externally
linkage with optimization enabled but bail because it doesn't make sense
for vararg functions.

PR18098.