[clang][dataflow] Use diagnosis API in optional checker

Followup to D127898. This patch updates `bugprone-unchecked-optional-access` to use the new `diagnoseCFG` function instead of just looking at the exit block.

A followup to this will update the optional model itself to use a noop lattice rather than redundantly computing the diagnostics in both phases of the analysis.

Reviewed By: ymandel, sgatev, gribozavr2, xazax.hun

Differential Revision: https://reviews.llvm.org/D128352