C++ coroutines on Clang 13 and libstdc++

Hi all,

I use folly in combination with clang 13 and libstdc++ on Ubuntu 20.10.
As far as I know, this combination should not support c++ coroutines as coroutines on clang require libc++.

However, [`Portability.h`](https://github.com/facebook/folly/blob/8bdd2efdd5f57e2e7480b9172e6619b3774f828f/folly/Portability.h#L554) sets `FOLLY_HAS_COROUTINES` to 1.

As far as I see the problem is that folly only checks if `(__has_include()` or `__has_include()` is valid.
This is true for my combination.

Thus, I think that we should extend this check to something like:

`(__has_include() && !defined(__clang__)) || (__has_include() && defined(__clang__))`

Best,
Philipp