Update to a new snapshot, now on the 7.0 line. 
Depend explicitly on python2.7. 
Install clang headers as they are now believed to work properly with
PR: 225315 , 225144 
Submitted by: sunpoet 
Differential Revision: https://reviews.freebsd.org/D13984
(Only the first 10 of 14 ports in this commit are shown above. )
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