[cmake] Include openmp with add_llvm_external_project

Summary:
Include projects/openmp into the build using add_llvm_external_project
instead of add_subdirectory.  This creates an option
LLVM_TOOL_OPENMP_BUILD that selects whether this project gets included
in an in-tree build.

Reviewers: beanz, AndreyChurbanov, jlpeyton

Subscribers: srhines, openmp-commits, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D30470

llvm-svn: 297185