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
Install a cmake config file. Copied exactly from how clang exports.
I also wasn't sure whether the canonical capitalization is "lld" or
"LLD". The project() is still calling this lld, but most places seemed
to capitalize it.