[libc++] Better handling for zero-sized types.

Zero-sized types are a GCC extension, also supported by Clang.
In theory it's already invalid to `delete` a void pointer or a
pointer-to-incomplete, so we shouldn't need any special code
to catch those cases; but in practice Clang accepts both
constructs with just a warning, and GCC even accepts `sizeof(void)`
with just a warning! So we must keep the static_asserts.
The hard errors are tested in `unique_ptr_dltr_dflt/*.compile.fail.cpp`.

In ranges::begin/end, check `sizeof >= 0` instead of `sizeof != 0`,
so as to permit zero-sized types while still disallowing incomplete
types.

Fixes #54100.

Differential Revision: https://reviews.llvm.org/D120633