Temporarily bump minimum compiler version

It's pretty hard to find a reliable list of which bots use which compiler version... so I'm going to commit this change which allows us to mandate the compilers required for C++14. This bump is what we've already agreed to do, so I'll use the list of failures to figure out which bots need to bump their compiler version. I'll revert the change in a few minutes.

The last discussion of this is here: http://lists.llvm.org/pipermail/llvm-dev/2019-August/134360.html

llvm-svn: 368252