Add support for Sphinx docx builder. Requires Sphinx docx builder to be
installed using pip install docxbuilder.
Added SPHINX_OUTPUT_DOCX CMake variable to control generation of docx
documentation. Defaults to OFF.
Added <project>_INSTALL_SPHINX_DOCX_DIR CMake variable to control where docx
files are installed. Defaults to share/doc/<project>/<project>.docx.
Documented new CMake variables in llvm/docs/CMake.rst.
Updated description of building documentation is lld/docs/sphinx_intro.rst,
lldb/docs/resources/build.rst, llvm/docs/README.txt, and
openmp/docs/README.txt,
Update clang/docs/CMakeLists.txt for Sphinx man and docx builders to ensure
.td generated .rst files are available to prevent warnings of missing rst files.
Minor CMake cleanups.
Added docx as an additional target for the documentation make.bat scripts.
Added docx as an additional target rule in the Makefile.sphinx files.