[Clang][Sema] Don't set instantiated from function when rewriting operator<=> (#91339)

The following snippet causes a crash:
```
template<typename T>
struct A 
{
    bool operator<=>(const A&) const requires true = default;
};

bool f(A<int> a) 
{
    return a != A<int>();
}
```
This occurs because during the rewrite from `operator<=>` to
`operator==`, the "pattern" `operator<=>` function is set as the
instantiated from function for the newly created `operator==` function.
This is obviously incorrect, and this patch fixes it.