openmp: Add support for OpenMP 5.2 linear clause syntax for C/C++

openmp: Add support for OpenMP 5.2 linear clause syntax for C/C++

The syntax for linear clause changed in 5.2, the original syntax
which is still valid is:
linear (var1, var2)
linear (var3, var4 : step1)
The 4.5 syntax with modifiers like:
linear (val (var5, var6))
linear (val (var7, var8) : step2)
is still supported in 5.2, but is deprecated there.
Instead, one can use a new syntax:
linear (var9, var10 : val)
linear (var11, var12 : step (step3), val)
As val, ref, uval or step (someexpr) can be valid expressions (and especially
in C++ can be const / constexpr / consteval), the spec says that
when the whole step expression is val (or ref or uval) or step ( ... )
then it is the new modifier syntax, one can use + 0 or 0 + or 1 * or * 1
or ()s to say it is the old step expression.
Also, 5.2 now allows val modifier to be specified even outside of declare simd
(but not the other modifiers).  I've implemented this for the new modifier
syntax only, the old one keeps the old restriction (which is why
OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER flag has been introduced).

2022-06-07  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* tree.h (OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER): Define.
	* tree-pretty-print.cc (dump_omp_clause) <case OMP_CLAUSE_LINEAR>:
	Adjust clause printing style depending on
	OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER.
gcc/c/
	* c-parser.cc (c_parser_omp_clause_linear): Parse OpenMP 5.2
	style linear clause modifiers.  Set
	OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER flag on the clauses when
	old style modifiers are used.
	* c-typeck.cc (c_finish_omp_clauses): Only reject linear clause
	with val modifier on simd or for if the old style modifiers are
	used.
gcc/cp/
	* parser.cc (cp_parser_omp_clause_linear): Parse OpenMP 5.2
	style linear clause modifiers.  Set
	OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER flag on the clauses when
	old style modifiers are used.
	* semantics.cc (finish_omp_clauses): Only reject linear clause
	with val modifier on simd or for if the old style modifiers are
	used.
gcc/fortran/
	* trans-openmp.cc (gfc_trans_omp_clauses): Set
	OMP_CLAUSE_LINEAR_OLD_LINEAR_MODIFIER on OMP_CLAUSE_LINEAR
	clauses unconditionally for now.
gcc/testsuite/
	* c-c++-common/gomp/linear-2.c: New test.
	* c-c++-common/gomp/linear-3.c: New test.
	* g++.dg/gomp/linear-3.C: New test.
	* g++.dg/gomp/linear-4.C: New test.
	* g++.dg/gomp/linear-5.C: New test.