Remove superceded warning warn_forgotten_module_header

This DefaultIgnore warning under -Wincomplete-module was firing on
any module map files that happened to be parsed (it's only supposed to
fire on headers), and it has been superceded by
-Wnon-modular-include-in-module anyway.

For compatibility, I rewired -Wincomplete-module to imply
-Wnon-modular-include-in-module.

llvm-svn: 221357