Add compile_flags.txt for clangd (C IDE) support

This file configures clangd (C Language Server for IDEs) for the GHC
project.
Please note that this only works together with Haskell Language Server,
otherwise .hie-bios/stage0/lib does not exist.

compile_flags.txt

+-xc
+-Irts
+-Irts/include
+-I.hie-bios/stage0/lib
+