[clang] [Clang] [Sema] Handle `this` in `__restrict`-qualified member functions properly (PR #83187)

[via cfe-commits]

Sirraide wrote:

> #11039

It does not fix this one because whereas GCC supports this, we don’t:
```c++
struct foo {
    void f();
};

void foo::f() __restrict {

}
```
because `__restrict` here is treated like `const`, so it has to be present on both the declaration and definition; however, GCC accepts this; should we do so as well?

The other two issues seem to be fixed by this, though.

https://github.com/llvm/llvm-project/pull/83187