[libc++abi] Avoid raw calls to assert() in libc++abi (#71121)

The runtimes now have a principled way of doing assertions in relation
to hardening, so we should use that instead of raw calls to assert()
inside libc++abi. This patch aims to maintain the behavior of the
demangler code when it is used from within LLVM by introducing a simple
DEMANGLE_ASSERT(...) macro that is then defined to the appropriate
assertion mechanism.

GitOrigin-RevId: c4779ea8e709a7bce7288988fabf1ba50e9c9477
7 files changed