[cxx-interop] Keep pulling in the entire libc++
libc++ recently split the `std` module into many top-level modules: https://github.com/llvm/llvm-project/commit/571178a21a8bc105bf86cf4bf92f842e07792e1a
Previously if a C++ module had `#include <iosfwd>`, importing that module in Swift would make the entire C++ stdlib visible from Swift, since it was a single top-level Clang module. After libc++ got split it doesn't automatically do so, but we need to preserve the current behavior for Swift users.
rdar://119270491
(cherry picked from commit 65eaafe7da5c25a49c8eec2079a88c33ce3cbed7)
