Dear Fortranners,

we missed certain intrinsics as being disallowed in constant expressions,
which lead to an ICE when these intrinsics were used in a specification
expression with an initializer.  The intrinsics in question are listed in
F2018:10.1.2.

As discussed in the PR, Steve recommended to omit TRANSFER from that list,
as it is special and might need separate treatment.  I also could not come
up with a case where TRANSFER should not have simplified to a constant and
we would run into an issue.  (We could leave that job to Gerhard... ;-).

However, in testing I encountered a case involving TRANSFER that is not
properly simplified, which seems orthogonal to the present case.  I would
like to handle this separately.  This case is mentioned in the testcase,
but commented out.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald