[clangd] Unbreak mac build differently 0c96a92d

This reverts b56e5f8a (and follow-up f6db8853) and instead
restores the state we had before 0c96a92d: ClangdMain.cpp
includes Features.inc before including Transport.h.

This is a bit ugly, but it matches the former state and making Transport.h
include Features.h means that xpc/ needs to be able to find the generated
Features.inc, wich is also a bit ugly.