[flang] Handle structure constructor in entity-decl

As with array constructors, we can encounter a decl-type-spec for a
structure-constructor while the decl-type-spec for the
type-declaration-stmt is still active. So here too we need to save and
restore the state.

In this case, it is done in DeclarationVisitor, so add
SetDeclTypeSpecState() to get and set the state.

Example where this occurs:
```
type(t(2)) :: x = t(2)()
```

Original-commit: flang-compiler/f18@aa5ca8d7b21d75ecd3c66a59589efd3ebb66af48
Reviewed-on: https://github.com/flang-compiler/f18/pull/249
Tree-same-pre-rewrite: false