Remove old fenv.h workaround for a historic clang driver bug

Tested and works fine with clang using libstdc++.

All indications are that this was fixed some time ago and isn't a problem with
any clang version we support.

I've added a note in PR6907 which is still open for some reason.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210485 91177308-0d34-0410-b5e6-96231b3b80d8