OpenMP: Handle more cases in user/condition selector

OpenMP: Handle more cases in user/condition selector

Tobias had noted that the C front end was not treating C23 constexprs
as constant in the user/condition selector property, which led to
missed opportunities to resolve metadirectives at parse time.
Additionally neither C nor C++ was permitting the expression to have
pointer or floating-point type -- the former being a common idiom in
other C/C++ conditional expressions.  By using the existing front-end
hooks for the implicit conversion to bool in conditional expressions,
we also get free support for using a C++ class object that has a bool
conversion operator in the user/condition selector.

gcc/c/ChangeLog
	* c-parser.cc (c_parser_omp_context_selector): Call
	convert_lvalue_to_rvalue and c_objc_common_truthvalue_conversion
	on the expression for OMP_TRAIT_PROPERTY_BOOL_EXPR.

gcc/cp/ChangeLog
	* cp-tree.h (maybe_convert_cond): Declare.
	* parser.cc (cp_parser_omp_context_selector): Call
	maybe_convert_cond and fold_build_cleanup_point_expr on the
	expression for OMP_TRAIT_PROPERTY_BOOL_EXPR.
	* pt.cc (tsubst_omp_context_selector): Likewise.
	* semantics.cc (maybe_convert_cond): Remove static declaration.

gcc/testsuite/ChangeLog
	* c-c++-common/gomp/declare-variant-2.c: Update expected output.
	* c-c++-common/gomp/metadirective-condition-constexpr.c: New.
	* c-c++-common/gomp/metadirective-condition.c: New.
	* c-c++-common/gomp/metadirective-error-recovery.c: Update expected
	output.
	* g++.dg/gomp/metadirective-condition-class.C: New.
	* g++.dg/gomp/metadirective-condition-template.C: New.