There's no real reason to use clang-cl on Windows, the clang driver
works just as well. This fixes a test which uses the -O0 flag, which was
recently removed from clang-cl to match MSVC, which lacks this flag.
While I'm here, remove the explicit -std=c++11 flag. Previously, this
flag was necessary when the default C++ standard was C++98. Now that the
default is C++14, this is no longer necessary. It's problematic on
Windows, because the Visual C++ standard library relies on C++14
features, and attempting to compile it with C++11 results in errors.
Rather than adding logic to conditionally set the standard to C++11 only
on non-Win, this flag can be removed.