[flang] Fix bug setting type from association expression

When an association selector is an expression, we get the type of the
associate-name from the type of the expression. If that is a derived
type, we were failing to get the derived type scope in copy of the
type that was made. That meant the component names were not resolved.

The fix is to copy the scope in the copy and move constructors of
`DerivedTypeSpec`. As a result, `DerivedTypeSpec::Instantiate` cannot
assume that the `scope_` is not set.

Original-commit: flang-compiler/f18@79eaa8c47fc0f1c1c71bf2760ec0a1e39e2a55ac
Reviewed-on: https://github.com/flang-compiler/f18/pull/281
Tree-same-pre-rewrite: false