Fortran: Fix incompatible types between INTEGER(8) and TYPE(c_ptr)

Fortran: Fix incompatible types between INTEGER(8) and TYPE(c_ptr)

In the context of an OpenMP declare variant directive, arguments of type C_PTR
are sometimes recognised as C_PTR in the base function and as INTEGER(8) in the
variant - or the other way around, depending on the parsing order.
This patch prevents such situation from turning into a compile error.

2023-10-20  Paul-Antoine Arras  <pa@codesourcery.com>
	    Tobias Burnus  <tobias@codesourcery.com>

gcc/fortran/ChangeLog:

	* interface.cc (gfc_compare_types): Return true if one type is C_PTR
	and the other is a compatible INTEGER(8).
	* misc.cc (gfc_typename): Handle the case where an INTEGER(8) actually
	holds a TYPE(C_PTR).

gcc/testsuite/ChangeLog:

	* gfortran.dg/c_ptr_tests_20.f90: New test, checking that INTEGER(8)
	and TYPE(C_PTR) are recognised as compatible.
	* gfortran.dg/c_ptr_tests_21.f90: New test, exercising the error
	detection for C_FUNPTR.