Gardening: Suppress -Winvalid-offsetof warnings in static asserts.

There cannot be undefined behavior at compile time, so these warnings can be
ignored when `offsetof` is used in a static assert:

```
warning: offset of on non-standard-layout type 'AsyncTask' [-Winvalid-offsetof]
```

Author: tshortli

include/swift/ABI/Task.h

@@ -736,9 +736,12 @@ static_assert(sizeof(AsyncTask) == NumWords_AsyncTask * sizeof(void*),
               "AsyncTask size is wrong");
 static_assert(alignof(AsyncTask) == 2 * alignof(void*),
               "AsyncTask alignment is wrong");
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-offsetof"
 // Libc hardcodes this offset to extract the TaskID
 static_assert(offsetof(AsyncTask, Id) == 4 * sizeof(void *) + 4,
               "AsyncTask::Id offset is wrong");
+#pragma clang diagnostic pop
 
 SWIFT_CC(swiftasync)
 inline void Job::runInFullyEstablishedContext() {

stdlib/public/Concurrency/TaskPrivate.h

@@ -865,8 +865,11 @@ struct AsyncTask::PrivateStorage {
 
 // It will be aligned to 2 words on all platforms. On arm64_32, we have an
 // additional requirement where it is aligned to 4 words.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-offsetof"
 static_assert(((offsetof(AsyncTask, Private) + offsetof(AsyncTask::PrivateStorage, StatusStorage)) % ACTIVE_TASK_STATUS_SIZE == 0),
    "StatusStorage is not aligned in the AsyncTask");
+#pragma clang diagnostic pop
 static_assert(sizeof(AsyncTask::PrivateStorage) <= sizeof(AsyncTask::OpaquePrivateStorage),
               "Task-private storage doesn't fit in reserved space");
 

stdlib/toolchain/Compatibility56/include/Concurrency/Task.h

@@ -605,9 +605,12 @@ static_assert(sizeof(AsyncTask) == NumWords_AsyncTask * sizeof(void*),
               "AsyncTask size is wrong");
 static_assert(alignof(AsyncTask) == 2 * alignof(void*),
               "AsyncTask alignment is wrong");
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-offsetof"
 // Libc hardcodes this offset to extract the TaskID
 static_assert(offsetof(AsyncTask, Id) == 4 * sizeof(void *) + 4,
               "AsyncTask::Id offset is wrong");
+#pragma clang diagnostic pop
 
 SWIFT_CC(swiftasync)
 inline void Job::runInFullyEstablishedContext() {