[flang] Restore checking for some optional values before use

Recent commits (2098ad7f and
15a9a72e) replaced usage of "o.value()"
on optionals with "*o".  Those optional values are expected to be
present -- but now, if it ever turns out that they're not,
compilation will proceed with garbage data rather than crashing
immediately (and more debuggably) with an uncaught exception.

Add asserts for presence to restore the previous level of safety.
(I could have revert these patches so as to resume used of .value()
but I didn't want to just have them get broken again.)

Differential Revision: https://reviews.llvm.org/D140340