[clang][modules] Avoid modules diagnostics for `__has_include()` (#71450)

After #70144 Clang started resolving module maps even for
`__has_include()` expressions. This had the unintended consequence of
emitting diagnostics around header misuse. These don't make sense if you
actually don't bring contents of the header into the importer, so should
be skipped for `__has_include()`. This patch moves emission of these
diagnostics out of `Preprocessor::LookupFile()` up into
`Preprocessor::LookupHeaderIncludeOrImport()`.