If you are on Debian or Ubuntu, it is now possible to build rustc with system LLVM that passes
make check. This is a howto.
You want LLVM 3.6 RC4 or later.
For Debian, you can install the official llvm-3.6-dev package. For Ubuntu, you can install from llvm.org APT repository. llvm.org APT repository is maintained by the same people who maintain the official Debian/Ubuntu packages.
$ ./configure --llvm-root=/usr/lib/llvm-3.6
Now if you run
make, no LLVM would be downloaded or compiled. System LLVM will be used instead.