[clang][Sema] Fix type of an statement expression ending with an atomic type (#119711)

When a statement expression's last statement is an atomic variable, GCC
and Clang disagree on the type of the expression. This can be made
apparent using `typeof` and forcing a diagnostic message:

```cpp
_Atomic int a = 0;
typeof(({a;})) x = "0";
```

* GCC complains about initializing `int` with `char*`
* Clang complains about initializing `_Atomic(int)` with a `char[2]`

Due to the type of the statement expression being deduced to be atomic,
we end with three implicit casts inside the `StmtExpr` on the AST:

* `LValueToRValue` -> `AtomicToNonAtomic`  ->  `NonAtomicToAtomic`

In some situations, this can end on an assertion inside
`IntExprEvaluator`, as reported in #106576.

With this patch, we now have two implicit casts, since the type of the
statement expression is deduced to be non-atomic:

* `LValueToRValue` -> `AtomicToNonAtomic`

This is consistent with the C standard (6.7.2.4, p4)

> The properties associated with atomic types are meaningful only for
expressions that are lvalues.

But a statement expression is an rvalue.

`IntExprEvaluator` assumptions are now satisfied and there is no
assertion error.
Additionally, the `typeof` trick mentioned above shows that the type is
consistently deduced between GCC and Clang.

Fixes #106576

---------

Co-authored-by: John McCall <rjmccall@gmail.com>