[flang] Fix bug for forward referenced type

A type name in an IMPLICIT declaration that was later used in a PARAMETER
statement caused problems because the default symbol scope had not yet been
initialized.  I avoided dereferencing in the situation where the default scope
was uninitialized and added a test that triggers the problem.

Also, once I corrected the bad dereference, the compiler was putting out
misleading error messages.  The underlying error us due to violating section
7.5.10, paragraph 4, which states:
  A structure constructor shall not appear before the referenced type is
  defined.

I fixed this by testing to see if a type that is used in a structure
constructor is forward referenced.

Differential Revision: https://reviews.llvm.org/D87535