[clang-tidy] Fix `misc-const-correctness` false positive on writes through assigned pointers (#216050)
`ExprPointeeResolve` did not resolve through `=`, so a write such as
`*(p = q) = 0` was not recognized as mutating `p`'s pointee and the
check suggested a `const` pointee that does not compile:
```cpp
void func(int *ptr1) {
int *ptr2 = ptr1;
int *ptr3 = ptr1;
*(ptr2 = ptr3) = 0; // warning: pointee of 'ptr2' can be declared 'const'
}
```
Godbolt: https://godbolt.org/z/3o8qnEKs3
An assignment yields an lvalue for the LHS holding the value of the RHS,
so
the dereferenced object is reachable through both operands. Resolve
through
both, like the existing additive case.
The RHS is already covered indirectly by AssignToNonConst, so the fix is
the LHS.
Fixes https://github.com/llvm/llvm-project/issues/216048.