openmp: Diagnose syntax mismatches between declare target and end declare target

openmp: Diagnose syntax mismatches between declare target and end declare target

OpenMP 5.1 says:
For any directive that has a paired end directive, including those with a begin
and end pair, both directives must use either the attribute syntax or the
pragma syntax.

The following patch enforces it with the only pair so far recognized in C++
(Fortran has many, but on the other side doesn't have attribute syntax).

While I initially wanted to use vec<bool, va_gc> *member; in there, that
unfortunately doesn't work, one gets linker errors and I guess it is fixable,
but for begin declare target we'll need a struct anyway to store device_type
etc.

2021-08-12  Jakub Jelinek  <jakub@redhat.com>

	* cp-tree.h (omp_declare_target_attr): New type.
	(struct saved_scope): Change type of omp_declare_target_attribute
	from int to vec<omp_declare_target_attr, va_gc> * and move it.
	* parser.c (cp_parser_omp_declare_target): Instead of
	incrementing scope_chain->omp_declare_target_attribute, push
	a struct containing parser->lexer->in_omp_attribute_pragma to
	the vector.
	(cp_parser_omp_end_declare_target): Instead of decrementing
	scope_chain->omp_declare_target_attribute, pop a structure
	from it.  Diagnose mismatching declare target vs.
	end declare target syntax.
	* semantics.c (finish_translation_unit): Use vec_safe_length
	and vec_safe_truncate on scope_chain->omp_declare_target_attributes.
	* decl2.c (cplus_decl_attributes): Use vec_safe_length
	on scope_chain->omp_declare_target_attributes.

	* g++.dg/gomp/attrs-12.C: New test.