openmp: Eliminate non-matching metadirective variants early in Fortran front-end

openmp: Eliminate non-matching metadirective variants early in Fortran front-end

This patch checks during parsing if a metadirective selector is both
resolvable and non-matching - if so, it is removed from further
consideration.  This is both more efficient, and avoids spurious
syntax errors caused by considering combinations of selectors that
lead to invalid combinations of OpenMP directives, when that
combination would never arise in the first place.

This exposes another bug - when metadirectives that are not of the
begin-end variety are nested, we might have to drill up through
multiple layers of the state stack to reach the state for the
next statement.  This is now fixed.

2022-02-11  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* omp-general.c (DELAY_METADIRECTIVES_AFTER_LTO): Check that cfun is
	non-null before derefencing.

	gcc/fortran/
	* decl.c (gfc_match_end): Search for first previous state that is not
	COMP_OMP_METADIRECTIVE.
	* gfortran.h (gfc_skip_omp_metadirective_clause): Add prototype.
	* openmp.c (match_omp_metadirective): Skip clause if
	result of gfc_skip_omp_metadirective_clause is true.
	* trans-openmp.c (gfc_trans_omp_set_selector): Add argument and
	disable	expression conversion if false.
	(gfc_skip_omp_metadirective_clause): New.

	gcc/testsuite/
	* gfortran.dg/gomp/metadirective-8.f90: New.