Support overriding the Doxygen & OCamldoc install directories, and provide a more FHS-compliant defaults for both of them. This extends r282240 that added this override for Sphinx-built documentation.
LLVM_INSTALL_DOXYGEN_HTML_DIR and LLVM_INSTALL_OCAMLDOC_HTML_DIR are added, to control the location where Doxygen-generated and OCamldoc-generated HTML docs are installed appropriately. They both specify CMake-style install paths, and therefore can either by relative to the install prefix or absolute.
The new defaults are subdirectories of share/doc/llvm, and replace the previous directories of 'docs/html' and 'docs/ocaml/html' that resulted in creating invalid '/usr/docs' that furthermore lacked proper namespacing for the LLVM package. The new defaults are consistent with the ones used for Sphinx HTML documentation, differing only in the last component. Since the 'html' subdirectory is already used for Sphinx docs, the 'doxygen-html' and 'ocaml-html' directories are used instead.