OpenMP/Fortran: Revamp handling of labels in metadirectives [PR122369,PR122508]

OpenMP/Fortran: Revamp handling of labels in metadirectives [PR122369,PR122508]

When a label is matched in the first statement after the end of a metadirective
body, it is bound to the associated region. However this prevents it from being
referenced elsewhere.
This patch fixes it by rebinding such labels to the outer region. It also
ensures that labels defined in an outer region can be referenced in a
metadirective body.

	PR fortran/122369
	PR fortran/122508

gcc/fortran/ChangeLog:
	* gfortran.h (gfc_rebind_label): Declare new function.
	* parse.cc (parse_omp_metadirective_body): Rebind labels to the outer
	region. Maintain a vector of metadirective regions.
	(gfc_parse_file): Initialise it.
	* parse.h (GFC_PARSE_H): Declare it.
	* symbol.cc (gfc_get_st_label): Look for existing labels in outer
	metadirective regions.
	(gfc_rebind_label): Define new function.
	(gfc_define_st_label): Accept duplicate labels in metadirective body.
	(gfc_reference_st_label): Accept shared DO termination labels in
	metadirective body.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/pr122369-1.f90: New test.
	* gfortran.dg/gomp/pr122369-2.f90: New test.
	* gfortran.dg/gomp/pr122369-3.f90: New test.
	* gfortran.dg/gomp/pr122369-4.f90: New test.
	* gfortran.dg/gomp/pr122508-1.f90: New test.
	* gfortran.dg/gomp/pr122508-2.f90: New test.