[flang] Allow DATA initialization of derived types w/ allocatable components

While one cannot of course statically initialize an allocatable component
of an instance of a derived type, its mere presence should not prevent
DATA initialization of the other nonallocatable components.  Semantics
was treating the existence of an allocatable component as a case of
"default initialization", which it is, but not one that should run
afoul of C877.  Add another Boolean argument to IsInitialized() to allow
for a more nuanced test.

Differential Revision: https://reviews.llvm.org/D119449