[cmake] Add support for multiple distributions

[cmake] Add support for multiple distributions

LLVM's build system contains support for configuring a distribution, but
it can often be useful to be able to configure multiple distributions
(e.g. if you want separate distributions for the tools and the
libraries). Add this support to the build system, along with
documentation and usage examples.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D89177