[Clang][OpenMP] Only check value if the expression is not instantiation dependent

Currently the following case fails:
```
template<typename Ty>
Ty foo(Ty *addr, Ty val) {
  Ty v;
#pragma omp atomic compare capture
  {
    v = *addr;
    if (*addr > val)
      *addr = val;
  }
  return v;
}
```

The compiler complains `addr` is not a lvalue. That's because when an expression
is instantiation dependent, we cannot tell if it is lvalue or not.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D135224