Mostly complete redo to the build of -devel LLVM ports:
- Switch to cmake.
- Combine all builds into devel/llvm-devel.
- Remove devel/libiomp5-devel
- Make lang/clang-devel a metaport so people can still find it.
Upgrade a snapshot shortly after the 3.7 branch point.
Sponsored by: DARPA, AFRL