[Sema] Fix PR28623.

In atomic builtins, we assumed that the LValue conversion on the first
argument would succeed. So, we would crash given code like:

```
void ovl(char);
void ovl(int);
__atomic_store_n(ovl, 0, 0);
```

This patch makes us not assume that said conversion is successful. :)

llvm-svn: 276232