[flang] Change WhyNotModifiable to return optional

One overload of WhyNotModifiable returned an optional message while the
other returns a unique_ptr. Change the latter to be consistent with the
former and other message-returning functions in this file.

Also, reorder the if clauses to reduce the level of indentation.

Original-commit: flang-compiler/f18@864e9cfc7ee98e3646052fdc757943011d641691
Reviewed-on: https://github.com/flang-compiler/f18/pull/1050
Tree-same-pre-rewrite: false