OpenMP: lvalue parsing for map/to/from clauses (C)

This patch adds support for parsing general lvalues ("locator list item
types") for OpenMP "map", "to" and "from" clauses to the C front-end,
similar to the previously-posted patch for C++.  Such syntax is permitted
for OpenMP 5.0 and above.  It was previously posted for mainline here:

  https://gcc.gnu.org/pipermail/gcc-patches/2022-December/609038.html

and for the og13 branch here:

  https://gcc.gnu.org/pipermail/gcc-patches/2023-June/623355.html

2024-01-11  Julian Brown  <julian@codesourcery.com>

gcc/c-family/
	* c-pretty-print.cc (c_pretty_printer::postfix_expression,
	c_pretty_printer::expression): Add OMP_ARRAY_SECTION support.

gcc/c/
	* c-parser.cc (c_parser_braced_init, c_parser_conditional_expression):
	Don't allow OpenMP array section.
	(c_parser_postfix_expression): Don't allow array section in statement
	expression.
	(c_parser_postfix_expression_after_primary): Add support for OpenMP
	array section parsing.
	(c_parser_expr_list): Don't allow OpenMP array section here.
	(c_parser_omp_variable_list): Change ALLOW_DEREF parameter to
	MAP_LVALUE.  Support parsing of general lvalues in "map", "to" and
	"from" clauses.
	(c_parser_omp_var_list_parens): Change ALLOW_DEREF parameter to
	MAP_LVALUE.  Update call to c_parser_omp_variable_list.
	(c_parser_oacc_data_clause): Update calls to
	c_parser_omp_var_list_parens.
	(c_parser_omp_clause_reduction): Use OMP_ARRAY_SECTION tree node
	instead of TREE_LIST for array sections.
	(c_parser_omp_target): Allow GOMP_MAP_ATTACH.
	* c-tree.h (c_omp_array_section_p): Add extern declaration.
	(build_omp_array_section): Add prototype.
	* c-typeck.cc (c_omp_array_section_p): Add flag.
	(mark_exp_read): Support OMP_ARRAY_SECTION.
	(build_omp_array_section): Add function.
	(build_external_ref): Tweak error path for OpenMP array sections.
	(handle_omp_array_sections_1): Use OMP_ARRAY_SECTION tree code instead
	of TREE_LIST.  Handle more kinds of expressions.
	(c_oacc_check_attachments): Use OMP_ARRAY_SECTION instead of TREE_LIST
	for array sections.
	(c_finish_omp_clauses): Use OMP_ARRAY_SECTION instead of TREE_LIST.
	Check for supported expression types.

gcc/testsuite/
	* gcc.dg/gomp/bad-array-section-c-1.c: New test.
	* gcc.dg/gomp/bad-array-section-c-2.c: New test.
	* gcc.dg/gomp/bad-array-section-c-3.c: New test.
	* gcc.dg/gomp/bad-array-section-c-4.c: New test.
	* gcc.dg/gomp/bad-array-section-c-5.c: New test.
	* gcc.dg/gomp/bad-array-section-c-6.c: New test.
	* gcc.dg/gomp/bad-array-section-c-7.c: New test.
	* gcc.dg/gomp/bad-array-section-c-8.c: New test.

libgomp/
	* libgomp.texi: C/C++ lvalues are supported now for map/to/from.
	* testsuite/libgomp.c-c++-common/ind-base-4.c: New test.
	* testsuite/libgomp.c-c++-common/unary-ptr-1.c: New test.