[clang] [analyzer][Solver] Early return if sym is concrete on assuming (PR #115579)

[Ding Fei via cfe-commits]

danix800 wrote:

For this testcase, two constrainst are collected on the path:

```c
(1) a + b + c == d
(2) b + c = 0
```

when assuming the third constraint `b == 0`, (1) or (2) is selected at random order for simplifying
eq class.

The fixedpoint simplification algorithm will recurse by `Re-evaluate an SVal with top-level State->assume logic`.
Early return in this PR would break this simplification.

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