Add a mode to clang-tblgen to generate reference documentation for warning and

Add a mode to clang-tblgen to generate reference documentation for warning and
remark flags. For now I'm checking in a copy of the built documentation, but we
can replace this with a placeholder (as we do for the attributes reference
documentation) once we enable building this server-side.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281192 91177308-0d34-0410-b5e6-96231b3b80d8