[libc++] Do not enable assertions in the dylib in the Apple cache

It turns out that all this time, we've actually been building without
assertions enabled in the dylib. This commit updates the Apple CMake
cache to make it consistent with reality.

libcxx/cmake/caches/Apple.cmake

@@ -2,7 +2,7 @@ set(CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "")
 set(CMAKE_POSITION_INDEPENDENT_CODE OFF CACHE BOOL "")
 
 set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
-set(LIBCXX_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LIBCXX_ENABLE_ASSERTIONS OFF CACHE BOOL "")
 
 set(LIBCXX_ABI_VERSION "1" CACHE STRING "")