[flang] Restore checking for some optional values before use

[flang] Restore checking for some optional values before use

Recent commits (2098ad7f00324ee0f2a6538f418a6f81dfdd2edb and
15a9a72ee68166c0cff3f036cacd3c82be66c729) 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