[CMake] Fix Libc++ Modules build.

When building the dylib, the C++ headers are fundamentally non-module.
They require special versions of the headers in order to provide C++03 and
legacy ABI definitions. This causes ODR issues when modules are enabled
during both the build and the usage of the libc++ headers.

This patch fixes the build error by disabling modules when building the
libc++ sources.

llvm-svn: 334220