[CMake] Fix ability to use LLVM_ENABLE_PROJECTS with LLVM_EXTERNAL_PROJECTS

LLVM r353148, changed the circumstances in which the project source directory variables are created to only create them for LLVM projects. This patch initializes the directory variables for projects specified in `LLVM_EXTERNAL_PROJECTS` as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354060 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed