[CMake] Make the runtimes directory work with bootstrap builds

[CMake] Make the runtimes directory work with bootstrap builds

This patch builds on clang r284648, and allows the runtime directory to make the bootstrap builds depend on the builtin libraries.

This patch also make the bootstrap build depend on configuring the other runtimes because the libcxx headers are copied during configuration. I have left a TODO in the code to remove that once I come up with a better solution.

llvm-svn: 284650