`dispatch_async_and_wait()` was introduced in macOS 10.14, which is
greater than our minimal deployment target. We need to forward declare
it as a "weak import" to ensure we generate a weak reference so the TSan
dylib continues to work on older systems. We cannot simply `#include
<dispatch.h>` or use the Darwin availability macros since this file is
multi-platform.
In addition, we want to prevent building these interceptors at all when
building with older SDKs because linking always fails.
Before:
```
➤ dyldinfo -bind ./lib/clang/12.0.0/lib/darwin/libclang_rt.tsan_osx_dynamic.dylib | grep dispatch_async_and_wait
__DATA __interpose 0x000F5E68 pointer 0 libSystem _dispatch_async_and_wait_f
```
After:
```
➤ dyldinfo -bind ./lib/clang/12.0.0/lib/darwin/libclang_rt.tsan_osx_dynamic.dylib | grep dispatch_async_and_wait
__DATA __got 0x000EC0A8 pointer 0 libSystem _dispatch_async_and_wait (weak import)
__DATA __interpose 0x000F5E78 pointer 0 libSystem _dispatch_async_and_wait (weak import)
```
This is a follow-up to D85854 and should fix:
https://reviews.llvm.org/D85854#2221529
Reviewed By: kubamracek
Differential Revision: https://reviews.llvm.org/D86103
686fe293e6