[analyzer][tests] Understand when diagnostics change between builds

Before the patch `SATest compare`, produced quite obscure results
when something about the diagnostic have changed (i.e. its description
or the name of the corresponding checker) because it was simply two
lists of warnings, ADDED and REMOVED.  It was up to the developer
to match those warnings, understand that they are essentially the
same, and figure out what caused the difference.

This patch introduces another category of results: MODIFIED.
It tries to match new warnings against the old ones and prints out
clues on what is different between two builds.

Differential Revision: https://reviews.llvm.org/D85311