[CMake] Support runtimes in distributions

Summary:
This patch adds a new internal variable
LLVM_RUNTIME_DISTRIBUTION_COMPONENTS which specifies distribution
components that are part of runtime projects, and thus should be exposed
from runtime configuraitons up into the top-level CMake configurations.

This is required for allowing runtime components to be included in
LLVM_DISTRIBUTION_COMPONENTS because we verify that the build and
install targets exist for every component specified for the
distribution.

Without this patch runtimes and builtins can only be included in
distributions in whole, not by component.

Reviewers: phosek

Reviewed By: phosek

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46705

llvm-svn: 332631