Fix parsing of substring refs in coarrays. [PR51815]

Fix parsing of substring refs in coarrays. [PR51815]

The parser was greadily taking the substring ref as an array ref because
an array_spec was present.  Fix this by only parsing the coarray (pseudo)
ref when no regular array is present.

gcc/fortran/ChangeLog:

	PR fortran/51815

	* array.cc (gfc_match_array_ref): Only parse coarray part of
	ref.
	* match.h (gfc_match_array_ref): Add flag.
	* primary.cc (gfc_match_varspec): Request only coarray ref
	parsing when no regular array is present.  Report error on
	unexpected additional ref.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr102532.f90: Fix dg-errors: Add new error.
	* gfortran.dg/coarray/substring_1.f90: New test.