[flang] Check for undefined derived types

It's possible to specify refer to an undefined derived type as the type of a
component of another derived type and then never define the type of the
component.  We were not detecting this situation.  To fix this, I
changed the value of isForwardReferenced_ in the symbol's
DerivedTypeDetails and checked for it when performing other derived type
checks.

I also had to record the fact that error messages were previously
emitted for the same problem in some cases so that I could avoid
duplicate messages.

I also added a test.

Differential Revision: https://reviews.llvm.org/D103714