Disable Modules when building the libc++ sources.
Libc++ will not build with modules enabled. In order to support an in-tree
libc++ when LLVM_ENABLE_MODULES is ON we need to explicitly disable the feature.
Unfortunately the libc++ sources are fundamentally non-modular. For example
iostream.cpp defines cout, cerr, wout, ... as char buffers instead of streams
in order to better control initialization/destruction. Not shockingly Clang
diagnoses this. Many other sources files define _LIBCPP_BUILDING_FOO macros to
provide definitions for normally inline symbols (See bind.cpp). Finally The
current module.map prohibits using <strstream> in C++11 so we can't build
I think I can fix most of these issues but until then just disable modules.