[flang] Fix misparsed structure constructor in data stmt

In a data statement like `data x / a(1) /`, `a(1)` may be an array
element or a structure constructor. It is parsed as an array element
so if it turns out `a` is a derived type it must be rewritten as a
strucutre constructor.

Original-commit: flang-compiler/f18@a2b2a330e7dbb8d719fdab9bb28921cf84f503ca
Reviewed-on: https://github.com/flang-compiler/f18/pull/1024