[TSan][Linux] Fix libdispatch interception macros compilation errors

Most libdispatch functions come in two variants: callbacks can be
specified via blocks or function pointers. Some of our interceptors for
the block variant actually forward to the function variant. However, on
Linux, `DECLARE_REAL(name)` has to appear before `REAL(name)`.

This patch reorders _f variant interceptors before _b variants
where possible and forward declares the _f variant in the remaining
cases (cyclic dependency between _f and _b interceptors).

Also rename macro to DISPATCH_INTERCEPT_ASYNC_F for better consistency.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D59067

llvm-svn: 355619