openmp: Improve handling of nested OpenMP metadirectives in C and C++

openmp: Improve handling of nested OpenMP metadirectives in C and C++

This patch fixes a misparsing issue when encountering code like:

  #pragma omp metadirective when {<selector_set>={...}: A)
    #pragma omp metadirective when (<selector_set>={...}: B)

When called for the first metadirective, analyze_metadirective_body would
stop just before the colon in the second metadirective because it naively
assumes that the '}' marks the end of a code block.

The assertion for clauses to end parsing at the same point is now disabled
if a parse error has occurred during the parsing of the clause, since some
tokens may not be consumed if a parse error cuts parsing short.

2022-02-18  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/c/
	* c-parser.c (c_parser_omp_construct): Move handling of
	PRAGMA_OMP_METADIRECTIVE from here...
	(c_parser_pragma): ...to here.
	(analyze_metadirective_body): Check that the bracket nesting level
	is also zero before stopping the adding of tokens on encountering a
	close brace.
	(c_parser_omp_metadirective): Modify function signature and update.
	Do not assert on remaining tokens if there has been a parse error.

	gcc/cp/
	* parser.c (cp_parser_omp_construct): Move handling of
	PRAGMA_OMP_METADIRECTIVE from here...
	(cp_parser_pragma): ...to here.
	(analyze_metadirective_body): Check that the bracket
	nesting level is also zero before stopping the adding of tokens on
	encountering a close brace.
	(cp_parser_omp_metadirective): Modify function signature and update.
	Do not assert on remaining tokens if there has been a parse error.

	gcc/testsuite/
	* c-c++-common/gomp/metadirective-1.c (f): Add test for
	improperly nested metadirectives.