cmake: Make the LLDB standalone build work for me

This allows me to generate a Visual Studio solution that builds LLDB if
you have standalone builds of LLVM and Clang lying around.
Unfortunately, you have to pass *four* CMake variables in, but it means
you don't have to take the extra step of installing Clang and LLVM to
some package prefix.

Hopefully this will generate a more usable XCode project too.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221413 91177308-0d34-0410-b5e6-96231b3b80d8