Cmake: allow using LLVM_EXTERNAL_PROJECTS with LLVM_ENABLE_PROJECTS

The current code iterates over the combination of LLVM_EXTERNAL_PROJECTS
and LLVM_ENABLE_PROJECTS, but then disables projects that are only in
the former. If a project is in LLVM_EXTERNAL_PROJECTS, it should be
enabled.

See also llvm-commits thread on r354060.

Differential revision: https://reviews.llvm.org/D62289

llvm-svn: 361751