Remove minimum toolchain soft-error

Summary:
Back in January I changed the minimum toolchain version required to build clang
and LLVM: D57264. Since then we've release LLVM 8, following
[our process](http://llvm.org/docs/DeveloperPolicy.html#toolchain)
it's therefore now a good time to remove the soft-error and officially deprecate
older toolchains. I tried this out last Tursday night to see if any bots
complained, and I saw no complaints. I also manually audited bots and didn't see
any bot that should break, but their toolchain information is unreliable and
some bots are offline.

Once this patch stick we'll move to C++14 as we've
[already agreed](http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html).

Subscribers: mgorny, jkorous, dexonsmith, llvm-commits, EricWF, thakis, chandlerc

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66188

llvm-svn: 368799