Recommit [libcxx] Never use <cassert> within libc++

It is my opinion that libc++ should never use `<cassert>`, including in the `dylib`.
This patch remove all uses of `assert` from within libc++ and replaces most of them with `_LIBCPP_ASSERT` instead.

Additionally this patch turn `LIBCXX_ENABLE_ASSERTIONS`  off by default,
because the standard library should not be aborting user programs unless explicitly asked to.

llvm-svn: 294107