[clang][dataflow][NFC] Fix comments related to widening.

The comments describing the API for analysis `widen` and the environment `widen`
were overly strict in the preconditions they assumed for the operation. In
particular, both assumed that the previous value preceded the current value in
the relevant ordering. However, that's not generally how widen operators work
and widening itself can violate this property. That is, when the previous value
is the result of a widening, it can easily be "greater" than the current value.

This patch updates the comments to accurately reflect the expectations.

Differential Revision: https://reviews.llvm.org/D140308