[Fuchsia] Add FUCHSIA_USE_MULTIPLE_DISTRIBUTIONS.

This flag causes the toolchain distribution to be built using LLVM
CMake's multiple distribution feature. The distribution* family of CMake
targets would be replaced with the toolchain-distribution* family.

This shouldn't otherwise affect the semantics of the build, but it sets
up the ability to split out the LLDB build from the main distribution
used by Fuchsia.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D146665