[flang] Correct defined assignment case (#142020)
When a generic ASSIGNMENT(=) has elemental and non-elemental specific
procedures that match the actual arguments, the non-elemental procedure
must take precedence. We get this right for generics defined with
interface blocks, but the type-bound case fails if the non-elemental
specific takes a non-default PASS argument.
Fixes https://github.com/llvm/llvm-project/issues/141807.
