Allow to opt into <experimental/coroutine> when <coroutine> is available

Summary:
- For Windows in xplat we use [Clang9/12 + MSVC 2017](https://www.internalfb.com/code/search?q=filepath%3Afbsource%2Ftools%2Fbuckconfigs%2Fcxx%2Fwindows%20repo%3Afbsource%20vs2017_15.9) and it's [<experimental/coroutine>](https://www.internalfb.com/phabricator/paste/view/P467028969)
- When upgrading to MSVC 2019 we also get [<coroutine>](https://www.internalfb.com/phabricator/paste/view/P467029007) in addition to  [<experimental/coroutine>](https://www.internalfb.com/phabricator/paste/view/P467029069)
- The MSVC2019 ```<coroutine>``` does not work with our current Windows [coroutine solution](https://fb.workplace.com/groups/rp.co.await/permalink/748274059391415/) / [D26877884](https://www.internalfb.com/diff/D26877884)
- Hence I am *re-using* an existing flag to opt-out of ```<coroutine>``` as MSVC 2019 left for backwards compatibility also ```<experimental/coroutine>```

Reviewed By: yfeldblum

Differential Revision: D32282445

fbshipit-source-id: d2eeb3057518e95135cf054ab6156dac5aa15065

folly/experimental/coro/Coroutine.h

@@ -27,7 +27,7 @@
 
 #if FOLLY_HAS_COROUTINES
 
-#if __has_include(<coroutine>)
+#if __has_include(<coroutine>) && !defined(LLVM_COROUTINES)
 #include <coroutine>
 #else
 #include <experimental/coroutine>
@@ -51,7 +51,7 @@ class exception_wrapper;
 
 namespace folly::coro {
 
-#if __has_include(<coroutine>)
+#if __has_include(<coroutine>) && !defined(LLVM_COROUTINES)
 namespace impl = std;
 #else
 namespace impl = std::experimental;