[flang] Handle array constructor in entity-decl

We can encounter a decl-type-spec for an array constructor while the
decl-type-spec for type-declaration-stmt is still active:
```
integer, allocatable :: a(:) = [integer :: 1, 2, 3]
```

To handle this, save the state of the current decl-type-spec while
processing the second one, then restore when done.

Original-commit: flang-compiler/f18@d125a0f612a320bd6faa103665ddfe0cb48199d9
Reviewed-on: https://github.com/flang-compiler/f18/pull/249
Tree-same-pre-rewrite: false