[flang] Accept NULL(mold=x) as constant component value in constant structure constructor

The predicate IsInitialDataTarget() was failing to return a correct true
result in the case of a reference to the intrinsic function NULL() with a
MOLD= argument.  Fix, and improve tests for "NULL()" elsewhere in semantics,
checking for an attribute set by intrinsics.cpp rather than the actual name.

Differential Revision: https://reviews.llvm.org/D119452