Sphinx-based clang man pages

Summary:
This diff introduces .rst files, Sphinx config, and a CMake target
for building clang man pages. This will deprecate the existing .pod-
based man page, and will integrate nicely with CMake. This diff does
not remove the existing man page; that will be done in a follow-up
once packagers have had a chance to react to the change.

For now, only clang(1) has been done; others can be added over time
by dropping additional files into the docs/CommandGuide directory.
The index page for CommandGuide has been copied from LLVM's
docs/CommandGuide.

The man page itself is mostly the same, with a few minor cosmetic
changes. The only major change is the SYNOPSIS section. I was unable
to get .rst/Sphinx produce the same style as in the existing man page.
Instead, I changed it to match the LLVM tools' relatively simple style.

To build the man pages, use the "docs-clang-man" target if building
with CMake. Otherwise, use "make -f Makefile.sphinx man".

Reviewers: cmatthews, silvas

Subscribers: dim, gaeke, beanz, cfe-commits

Differential Revision: http://reviews.llvm.org/D10562

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241037 91177308-0d34-0410-b5e6-96231b3b80d8