[clang] [analyzer] Fix null-pointer dereference in PthreadLockChecker (PR #210912)

[Balázs Benics via cfe-commits]

steakhal wrote:

> It wraps `simplifyToSVal` that does not discard concrete values. I'll try to use `simplifyToSVal` in the 3 places `simplify` is used in and handle concrete values there

I looked at the code and at the time the idea was that we want to constant arithmetic on symbols by folding those constants: `((x + 2) + 3) => (x + 5)`  I guess we didn't account for folding it into a constant.

PS: I was just about to hit my reply when I noticed you pushed. I'll have a look at the recent changes tomorrow.

https://github.com/llvm/llvm-project/pull/210912