openmp: Add support for resolving metadirectives during parsing and Gimplification

openmp: Add support for resolving metadirectives during parsing and Gimplification

This adds support for resolving metadirectives according to the OpenMP 5.1
specification.  The variants are sorted by score, then gathered into a list
of dynamic replacement candidates.  The metadirective is then expanded into
a sequence of 'if..else' statements to test the dynamic selector and execute
the variant if the selector is satisfied.

If any of the selectors in the list are unresolvable, GCC will give up on
resolving the metadirective and try again later.

2022-01-25  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* gimplify.c (expand_omp_metadirective): New.
	* omp-general.c: Include tree-pretty-print.h.
	(DELAY_METADIRECTIVES_AFTER_LTO): New macro.
	(omp_context_selector_matches): Delay resolution of selectors.  Allow
	non-constant expressions.
	(omp_dynamic_cond): New.
	(omp_dynamic_selector_p): New.
	(sort_variant): New.
	(omp_get_dynamic_candidates): New.
	(omp_resolve_metadirective): New.
	(omp_resolve_metadirective): New.
	* omp-general.h (struct omp_metadirective_variant): New.
	(omp_resolve_metadirective): New prototype.

	gcc/c-family/
	* c-omp.c (c_omp_expand_metadirective_r): New.
	(c_omp_expand_metadirective): New.