clangd: fix the build with XPC

`Transport.h` does not include `Features.inc`.  However, since it is used in a
subdirectory, it cannot directly include the header as it is not available.
Include `Features.inc` in `ClangdLSPServer.h` prior to the inclusion of
`Transport.h` which will provide the interfaces in `ClangdMain.cpp` where the
symbol `newXPCTransport` will not be defined due to it being preprocessed away
since the configuration is not passed along to the initial inclusion.

llvm-svn: 358103