[libc++] Audit all uses of _LIBCPP_ASSERT and _LIBCPP_DEBUG_ASSERT

I audited all uses of _LIBCPP_ASSERT to make sure that we only used it
for "basic assertions", i.e. assertions with constant-time conditions.
I also audited all uses of _LIBCPP_DEBUG_ASSERT to make sure we used it
only for debug-mode assertions, and in one case had to change for
_LIBCPP_ASSERT instead.

As a fly-by, I also changed a couple of tests against nullptr or 0 to
be more explicit.

After this patch, all uses of _LIBCPP_ASSERT should be with constant-time
conditions, and all uses of _LIBCPP_DEBUG_ASSERT should be with conditions
that we only want to check when the debug mode is enabled.

Differential Revision: https://reviews.llvm.org/D122395