[flang] Relax the implementation of constraint C1302. (flang-compiler/f18#805)

* Relax the implementation of constraint C1302.

When a list of format items can be unambiguously partitioned into individual items even though one or more otherwise required comma separators are omitted, generate a warning rather than an error.

Fixes flang-compiler/f18#703

Original-commit: flang-compiler/f18@79c0731137e1092c26c4dec7c12113c52a72effd
Reviewed-on: https://github.com/flang-compiler/f18/pull/805