[flang] Intrinsic assignment of distinct but "same" derived types

Subclause 7.5.2.4 lists conditions under which two distinct derived
types are to be considered the same type for purposes of argument
association, assignment, and so on.  These conditions are implemented
in evaluate::IsTkCompatibleWith(), but assignment semantics doesn't
use it for testing for intrinsic assignment compatibility.  Fix that.

Differential Revision: https://reviews.llvm.org/D117621