docs: Initial Sphinx setup for Clang.

I don't think this will be visible just yet on <clang.llvm.org/docs/>
since I don't think that the necessary server-side setup has taken
place.

Don't shoot me over the theme. I don't want to duplicate LLVM's theme
into the clang repo at the moment, so I just used one of Sphinx's
default themes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170042 91177308-0d34-0410-b5e6-96231b3b80d8