Sink the autoconf check for sufficiently modern host toolchain below the
enable flag that selects the C++ standard library to use with the host
toolchain. Otherwise we end up testing the wrong config.

I'm not really happy about this placement, but its pragmatic and should
unblock the Apple builders.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199325 91177308-0d34-0410-b5e6-96231b3b80d8