OpenACC 2.7: Connect readonly modifier to points-to analysis

This patch links the readonly modifier to points-to analysis.

In front-ends, firstprivate pointer clauses are marked with
OMP_CLAUSE_MAP_POINTS_TO_READONLY set true, and later during lowering the
receiver side read of pointer has VAR_POINTS_TO_READONLY set true, which later
directs SSA_NAME_POINTS_TO_READONLY_MEMORY set to true during SSA conversion.

SSA_NAME_POINTS_TO_READONLY_MEMORY is an already existing flag connected with
alias oracle routines in tree-ssa-alias.cc, thus making the readonly-modifier
effective in hinting points-to analysis.

Currently have one testcase c-c++-common/goacc/readonly-2.c where we can
demonstrate 'readonly' can avoid a clobber by function call.

This patch is ported from upstream submission:
https://gcc.gnu.org/pipermail/gcc-patches/2024-April/648728.html

gcc/c-family/ChangeLog:

	* c-omp.cc (c_omp_address_inspector::expand_array_base):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.
	(c_omp_address_inspector::expand_component_selector): Likewise.

gcc/fortran/ChangeLog:

	* trans-openmp.cc (gfc_trans_omp_array_section):
	Set OMP_CLAUSE_MAP_POINTS_TO_READONLY on pointer clause.

gcc/ChangeLog:

	* gimple-expr.cc (copy_var_decl): Copy VAR_POINTS_TO_READONLY
	for VAR_DECLs.
	* omp-low.cc (lower_omp_target): Set VAR_POINTS_TO_READONLY for
	variables of receiver refs.
	* tree-pretty-print.cc (dump_omp_clause):
	Print OMP_CLAUSE_MAP_POINTS_TO_READONLY.
	(dump_generic_node): Print SSA_NAME_POINTS_TO_READONLY_MEMORY.
	* tree-ssanames.cc (make_ssa_name_fn): Set
	SSA_NAME_POINTS_TO_READONLY_MEMORY if DECL_POINTS_TO_READONLY is set.
	* tree.h (OMP_CLAUSE_MAP_POINTS_TO_READONLY): New macro.
	(VAR_POINTS_TO_READONLY): New macro.

gcc/testsuite/ChangeLog:

	* c-c++-common/goacc/readonly-1.c: Adjust testcase.
	* c-c++-common/goacc/readonly-2.c: New testcase.
	* gfortran.dg/goacc/readonly-1.f90: Adjust testcase.
	* gfortran.dg/pr67170.f90: Likewise.

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