[CMake] Teach build system to build/test compiler-rt with a just-built Clang

With this change, one may set LLVM_BUILD_EXTERNAL_COMPILER_RT option
to build compiler-rt libraries with just-built Clang.

  make compiler-rt
in the build tree will build all compiler-rt libraries with just-built Clang
and copy them to the proper location in the Clang resource directory.

  make check-compiler-rt
will run the compiler-rt test suite using just-built Clang and runtime
libraries.

The goal is to make LLVM_BUILD_EXTERNAL_COMPILER_RT the default, so that
we can always build compiler-rt libraries with Clang, not the host compiler,
and for all the platforms Clang can target.