Remove Bogus Macro Assert in LUT Building
This assertion assumes that only visibility directives for builtin-in
macros find their way here. However, the presence of textual headers
in modules means any macro directive can wind up here since clang will
report that it is currently not building a module.
It's been half a decade since this code did anything useful with
macro directives in textual headers, so just leave things be.
rdar://91768988
