Switch LangOptions over to a .def file that describes header of the

Switch LangOptions over to a .def file that describes header of the
language options. Use that .def file to declare the LangOptions class
and initialize all of its members, eliminating a source of annoying
initialization bugs.

AST serialization changes are next up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139605 91177308-0d34-0410-b5e6-96231b3b80d8