[runtimes] Add a mechanism to use cache files for a runtimes build

Projects like libc++ include their own cache files, and it's convenient
to just be able to reuse those cache files as part of a runtimes build
instead of having to duplicate the settings inside a special runtimes
cache file (which will inevitably get out of sync).

I'm not completely happy about overloading the existing argument passing
behavior based on the argument name, but I also couldn't think of a good
alternative. Suggestions are welcome.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D158791