Added Sphinx documentation generation to CMake build system.

The option LLVM_ENABLE_SPHINX option enables the "docs-llvm-html",
"docs-llvm-man" targets but does not build them by default. The
following CMake options have been added that control what targets are
made available

SPHINX_OUTPUT_HTML
SPHINX_OUTPUT_MAN

If LLVM_BUILD_DOCS is enabled then the enabled docs-llvm-* targets will
be built by default and if ``make install`` is run then docs-llvm-html
and docs-llvm-man will be installed (tested on Linux only).

The add_sphinx_target function is in its own file so it can be included
by other projects that use Sphinx for their documentation.

Patch by Daniel Liew <daniel.liew@imperial.ac.uk>!