[Testing] Workaround libcxx bug when OS is "none"

If clang is configured to use libcxx as the default C++ standard
library, then using "none" for the OS in the target triple will cause
libcxx to #error needlessly. Passing -nostdinc++ is a workaround for
these tests. See also: https://reviews.llvm.org/D68075

Please note: this workaround will probably exist for a few years until
the installed version of libcxx is updated.

llvm-svn: 372949