Fix missing header guard warning for a standalone header being always present

Meaning a header that is not included in any other source file in your project.
Such a header would always have the "missing include guard" warning, regardless
if guards are present or not.

This is caused by clang_isFileMultipleIncludeGuarded returning 0 (not guarded) for
standalone header. I added a clang_Location_isFromMainFile check, which checks if
the "source location" is in the main file, which (if we only have a header) is true.