Attempt to unbreak Windows configuration.

Although libc++ doesn't yet support Windows we still have Windows
builders to track our progress.

Currently the clang-cl configuration seems broken because it doesn't
support -std=c++11 and instead requires /std:c++11. This patch attempts
to fix this.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@343431 91177308-0d34-0410-b5e6-96231b3b80d8