[Patch, Fortran] PR44350 - add constraint check for BLOCK DATA

[Tobias Burnus]

A rather simple patch, which tries to implement Fortran 2008's C1116 
(see also PR for the wording).

While creating the patch, I found a reject-valid issue, which is now 
tracked as PR fortran/58857.

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

Tobias
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