Install clang-3.8 via mason binary

[springmeyer]

We needed to disable clang-3.8 builds in e7159ad due to LLVM apt servers going offline (http://lists.llvm.org/pipermail/llvm-foundation/2016-June/000025.html).

This brings back testing of clang-3.8 via a mason package downloaded from s3, which should be fast and stable.

Bonus: this package supports -fsanitize=<things> like address and integer.

[springmeyer]

This is working now after fixing the clang-3.8 builds to be proper Release builds and disabling openmp, which is not working yet in the clang-3.8 binaries.

As far as openmp, we have a few routes:

• 1. keep the above patch that disables openmp
• 2. rebuild clang-3.8 binaries again to fully remove partial/broken openmp support

I am out of the office till next Monday. When I'm back I can take a look at 2).

[springmeyer]

2. rebuild clang-3.8 binaries again to fully remove partial/broken openmp support

Was able to do this today, so I reverted the disabling of openmp, and the builds are working now. This is ready to merge once travis is all green.

[daniel-j-h]

Interested in how OpenMP support is broken in LLVM 3.8. Care to elaborate?

[springmeyer]

Interested in how OpenMP support is broken in LLVM 3.8. Care to elaborate?

@daniel-j-h it was compiled into the binary but then programs that linked it needed to load the libomp.so library at runtime. That would not work with a mason provided clang++ since the libomp.so is not in the default shared library paths. We could get it working by setting LD_LIBRARY_PATH to the custom libomp.so location. But since clang++ builds have not been using omp previously I did not want to address that in this pull.