Fortran: Diagnose default-initialized pointer/allocatable dummies

Fortran: Diagnose default-initialized pointer/allocatable dummies

TS29113 changed what was then C516 in the 2010 Fortran standard (now
C1557 in F2018) from disallowing all of pointer, allocatable, and
optional attributes on dummy arguments to BIND(C) functions, to
disallowing only pointer/allocatable with default-initialization.
gfortran was previously failing to diagnose violations of this
constraint.

2021-09-23  Sandra Loosemore  <sandra@codesourcery.com>

	PR fortran/101320

gcc/fortran/
	* decl.c (gfc_verify_c_interop_param): Handle F2018 C1557,
	aka TS29113 C516.

gcc/testsuite/
	* gfortran.dg/c-interop/c516.f90: Remove xfails.  Add more
	tests.