OpenMP 5.0: Allow multiple clauses mapping same variable

OpenMP 5.0: Allow multiple clauses mapping same variable

This is a merge of:
https://gcc.gnu.org/pipermail/gcc-patches/2020-December/562081.html

This patch now allows multiple clauses on the same construct to map
the same variable, which was not valid in OpenMP 4.5, but now allowed
in 5.0.

This may possibly reverted/updated when a final patch is approved
for mainline.

2021-02-01  Chung-Lin Tang  <cltang@codesourcery.com>

gcc/cp/ChangeLog:

	* semantics.cc (finish_omp_clauses):  Adjust to allow duplicate
	mapped variables for OpenMP.

gcc/ChangeLog:

	* omp-low.cc (install_var_field): Add new 'tree key_expr = NULL_TREE'
	default parameter. Set splay-tree lookup key to key_expr instead of
	var if key_expr is non-NULL. Adjust call to install_parm_decl.
	Update comments.
	(scan_sharing_clauses): Use clause tree expression as splay-tree key
	for map/to/from and OpenACC firstprivate cases when installing the
	variable field into the send/receive record type.
	(maybe_lookup_field_in_outer_ctx): Add code to search through
	construct clauses instead of entirely based on splay-tree lookup.
	(lower_oacc_reductions): Adjust to find map-clause of reduction
	variable, then create receiver-ref.
	(lower_omp_target): Adjust to lookup var field using clause expression.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/clauses-2.c: Adjust testcase.