[Patch, Fortran] PR57530 - fix rejects valid with gfc_type_compatible

[Tobias Burnus]

A TYPE is type compatible with a CLASS if both have the same declared type.

Or in words of the standard (cf. PR):
"A nonpolymorphic entity is type compatible only with entities of the 
same declared type. A polymorphic entity that is not an unlimited 
polymorphic entity is type compatible with entities of the same declared 
type or any of its extensions." (F2008, 4.3.1.3).

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias
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