[CMake] Honor LLVM_EXTERNAL_<proj>_SOURCE_DIR

When LLVM_ENABLE_PROJECTS is set, CMake assumes the project
directories are all side-by-side. This is not always the case and
there's no reason to expect it if LLVM_EXTERNAL_<proj>_SOURCE_DIR is
set. Honor that setting if it exists and allow the build configuration
to continue.

Differential Revision: https://reviews.llvm.org/D49672

llvm-svn: 354693