[TSan][libdispatch] Guard test execution on old platforms

`dispatch_async_and_wait()` was introduced in macOS 10.14.  Let's
forward declare it to ensure we can compile the test with older SDKs and
guard execution by checking if the symbol is available.  (We can't use
`__builtin_available()`, because that itself requires a higher minimum
deployment target.)  We also need to specify the `-undefined
dynamic_lookup` compiler flag.

Differential Revision: https://reviews.llvm.org/D85995