[clang] [StaticAnalyzer] early return if sym is concrete on assuming (PR #115579)

[Balazs Benics via cfe-commits]

https://github.com/steakhal commented:

> Thanks for the commit, I'm satisfied with it :)
> 
> I actually like that these two related changes (the checker change and the constraint manager improvement) are handled together in a single commit -- this way somebody who browses the commit log can directly see the "other half" of the change (without following cumbersome links through "this commit is mentioned in the commit message of that one" or opening this github review). Also, the checker change is so trivial (+2 lines) that the full combined commit is still small enough to be easily understood.

I agree.

The patch looks good to me. I made some recommendations for improving the tests.

I have one question. In the StdLibraryFn checker you added a test for `isSink()`.
I thought that the crash was that some `assume(..., true|false)` returned a null State, that we dereferenced. Where was that assume call and how did we unconditionally dereference it?

https://github.com/llvm/llvm-project/pull/115579