[CMake] Pass LLVM_EXTERNAL_*_SOURCE_DIR variables to subsequent stages

For multi-stage builds we need to pass any overridden source directory variables. Without passing these the subsequent stages won't find the project sources.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260341 91177308-0d34-0410-b5e6-96231b3b80d8