[flang] Derived type structural equivalence (#69376)

F'202X 7.5.2.4 describes conditions under which two derived type
definitions are to be considered equivalent. These rules are already
implemented in Evaluate/type.cpp but not exposed for general use;
rearrange the code a little so that the compatibility checking of
separate module procedure interfaces and explicit definitions can use it
to avoid emitting a bogus error message.

Fixes https://github.com/llvm/llvm-project/issues/67946.