[CMake] Allow LLVM_EXTERNAL_<proj>_SOURCE_DIR to be overridden if it is

empty.

This makes adding projects to LLVM_ENABLE_PROJECTS possible.
Also its type should be PATH.

https://bugs.llvm.org/show_bug.cgi?id=42698

Reviewers: beanz, greened, chapuni

Reviewed by: beanz

Differential Revision: https://reviews.llvm.org/D65045

llvm-svn: 367144