[flang] Merge pull request flang-compiler/f18#322 from flang-compiler/pmk-struct-constr-disambig

Fix two ambiguous parsing cases by patching parse tree in expression semantics

Original-commit: flang-compiler/f18@d4807f136419b075adcaf38b0129ba7e79c994df
Reviewed-on: https://github.com/flang-compiler/f18/pull/322