Ensure that headers initially marked CXX can be later marked mixed.

When a header was marked CXX, it wouldn't be parsed for CPP_IDENTIFIERS and
then wouldn't be opened or read past that point. This meant that it could
never be marked as a mixed mode header, which was a bug.

Use a new map to determine which headers are mixed mode and scan all headers
(not just C ones) for CPP_IDENTIFIERS. The mixed mode language will be
determined at the call site of
_psq_get_absolute_path_to_header_file_language