[flang] Fix bug flang-compiler/f18#376, internal error when correcting...

[flang] Fix bug flang-compiler/f18#376, internal error when correcting misparse of structure constructor

Original-commit: flang-compiler/f18@f18a94ef9690c412907c733ac08be6736ea4eaad
Reviewed-on: https://github.com/flang-compiler/f18/pull/378
Tree-same-pre-rewrite: false