[flang] Adjust productions in grammar.h so that construct<> invocations span...

[flang] Adjust productions in grammar.h so that construct<> invocations span what they cover (more to come)

Original-commit: flang-compiler/f18@ba5c2368d1c8b40d36746d16a2ee848ecfc5152c
Reviewed-on: https://github.com/flang-compiler/f18/pull/69
Tree-same-pre-rewrite: false