Take another crack at stabilizing the emission order of analyzer

diagnostics without using FoldingSetNodeIDs.  This is done
by doing a complete recursive comparison of the PathDiagnostics.

Note that the previous method of comparing FoldingSetNodeIDs did
not end up relying on unstable things such as pointer addresses, so
I suspect this may still have some issues on various buildbots because
I'm not sure if the true source of non-determinism has been eliminated.
The tests pass for me, so the only way to know is to commit this change
and see what happens.

llvm-svn: 163489