This is a bit of an interesting upgrade to LLVM. Upstream LLVM has started using C++11 features, so they require a C++11 compiler to build. I've updated all the bots to have a C++11 compiler, and they appear to be building LLVM successfully:
* Linux bots - I added gcc/g++ 4.7 (good enough)
* Android bots - same as the linux ones
* Mac bots - I installed the most recent command line tools for Lion which gives us clang 3.2, but LLVM wouldn't build unless it was explicitly asked to link to `libc++` instead of `libstdc++`. This involved tweaking `mklldeps.py` and the `configure` script to get things to work out
* Windows bots - mingw-w64 has gcc 4.8.1 which is sufficient for building LLVM (hurray!)
* BSD bots - I updated FreeBSD to 10.0 which brought with it a relevant version of clang.
The largest fallout I've seen so far is that the test suite doesn't work at all on FreeBSD 10. We've already stopped gating on FreeBSD due to #13427 (we used to be on freebsd 9), so I don't think this puts us in too bad of a situation. I will continue to attempt to fix FreeBSD and the breakage on there.
The LLVM update brings with it all of the recently upstreamed LLVM patches. We only have one local patch now which is just an optimization, and isn't required to use upstream LLVM. I want to maintain compatibility with LLVM 3.3 and 3.4 while we can, and this upgrade is keeping us up to date with the 3.5 release. Once 3.5 is release we will in theory no longer require a bundled LLVM.
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