[CMake] Add support for building the llvm test-suite as part of an LLVM build using clang and lld

Summary:
This patch adds a new CMake module for working with ExternalProjects. This wrapper for ExternalProject supports using just-built tools and can hook up dependencies properly so that projects get cleared out.

The example usage here is for the llvm test-suite. In this example, the test-suite is setup as dependent on clang and lld if they are in-tree. If the clang or lld binaries change the test-suite is re-configured, cleaned, and rebuilt.

This cleanup and abstraction wrapping ExternalProject can be extended and applied to other runtime libraries like compiler-rt and libcxx.

Reviewers: samsonov, jroelofs, rengolin, jmolloy

Subscribers: jmolloy, llvm-commits

Differential Revision: http://reviews.llvm.org/D14513

llvm-svn: 252747