Including <dispatch/dispatch.h> and <Blocks.h> transitively pulls in
other system headers. Let's try to avoid that.
Blocks.h: compiler-rt already includes a blocks runtime. Just use the
header file that comes with it.
dispatch.h: Declare the bare minimum required for our implementation,
i.e., everything needed to define the interceptors, but not the
interceptors themselves. See tsan_dispatch_defs.h. I spotted a few other
places in compile-rt, where we declare libdispatch types. Maybe this
file can be moved to sanitizer_common if we deem it useful enough.
tsan_libdispatch.cc now compiles on Linux/Clang (requires support for
-fblocks). Linking still requires some manual configuration.