[CMake] Create an install-distribution target driven by LLVM_DISTRIBUTION_COMPONENTS

The idea here is to provide a customizable install target that only depends on building the things you actually want to install. It relies on each component being installed having an auto-generated install-${component}, which in turn depends only on the target being installed.

This is fundamentally a workaround for the fact that CMake generates build files which have their "install" target depend on the "all" target. This results in "ninja install" building a bunch of unneeded things.

llvm-svn: 261681