openmp: Add support for __has_attribute(omp::directive) and __has_attribute(omp:...

openmp: Add support for __has_attribute(omp::directive) and __has_attribute(omp::sequence)

Now that the C++ FE supports these attributes, but not through registering
them in the attributes tables (they work quite differently from other
attributes), this teaches c_common_has_attributes about those.

2021-07-23  Jakub Jelinek  <jakub@redhat.com>

	* c-lex.c (c_common_has_attribute): Call canonicalize_attr_name also
	on attr_id.  Return 1 for omp::directive or omp::sequence in C++11
	and later.

	* c-c++-common/gomp/attrs-1.c: New test.
	* c-c++-common/gomp/attrs-2.c: New test.
	* c-c++-common/gomp/attrs-3.c: New test.