libcpp: Diagnose __has_include outside of preprocessor directives [PR93545]

libcpp: Diagnose __has_include outside of preprocessor directives [PR93545]

The standard says http://eel.is/c++draft/cpp.cond#7.sentence-2 that
__has_include can't appear at arbitrary places in the source.  As we have
not recognized __has_include* outside of preprocessing directives in the
past, accepting it there now would be a regression.  The patch does still
allow it in #define if it is then used in preprocessing directives, I guess
that use isn't strictly valid either, but clang seems to accept it.

2020-02-04  Jakub Jelinek  <jakub@redhat.com>

	* macro.c (builtin_has_include): Diagnose __has_include* use outside
	of preprocessing directives.

	* c-c++-common/cpp/has-include-1.c: New test.
	* c-c++-common/cpp/has-include-next-1.c: New test.
	* c-c++-common/gomp/has-include-1.c: New test.