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.

Differential Revision: https://reviews.llvm.org/D24935

llvm-svn: 282536