[TSan][libdispatch] Ensure TSan dylib works on old systems

`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

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