llvm: various improvements.

* Use consistent ver function for version suffix so that release and
  HEAD shares the same version suffix.
* Use the built clang to build libc++.
  - Building libc++ requires C++11 support, which is not present in
    older compilers like Apple gcc-4.2/llvm-gcc-4.2.
  - Require "--with-clang" when specifying "--with-libcxx".
* Drop "-lc++" from caveats. Unnecessary when using clang++.
* llvm34: Migrate to resource.
* llvm33: Add head links.

Closes #219.