[clang][dataflow][NFC] Expand comments on losing values in optional checker.

While working on the ongoing migration to strict handling of value
categories (see https://discourse.llvm.org/t/70086), I ran into issues related
to losing the value associated with an optional.

This issue is hinted at in the existing comments, but the issue didn't become
sufficiently clear to me from those, so I thought it would be worth capturing
more details, along with ideas for how this issue might be fixed.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D152369