[libcxx] r243574 - Recommit r243503 "[libcxx] Cleanup CMake configuration and integrate with LLVM"

[Eric Fiselier]

> I'm not sure why the behavior has changed, though.

libc++ used to specify '-Wno-error' whenever LIBCXX_ENABLE_WERROR was
OFF. This patch removed that.
I'll put it back in for now but I don't think libc++ should be
suppressing -Werror.