[libc++abi] Disable libc++ assertions when building cxa_demangle

[arichardson]

Summary

Debug builds of libc++abi with libc++ assertions currently fail due to a
missing reference to __libcpp_verbose_abort(). This fixes the linker errors
by disabling the libc++-internal assertions while building this file.
I'm not convinced this is the best approach for fixing this problem, maybe
we should just provide a stub implementation of __libcpp_verbose_abort()
in this file instead.

Fixes: https://github.com/llvm/llvm-project/issues/58117

[ldionne]

Instead, I think I would add target_compile_definitions to cxxabi_shared and cxxabi_static.

[urnathan]

I'ma gonna replace the call to lower_bound in the demangler. https://github.com/llvm/llvm-project/issues/58117

[arichardson]

Abandoning in favour of @urnathan's version

libcxxabi/src/cxa_demangle.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // FIXME: (possibly) incomplete list of features that clang mangles that this
 // file does not yet support:
 //   - C++ modules TS
 
+// We have to avoid any references to the libc++ dylib, so we
+// explicitly disable assertions here (as that could result in
+// a call to __libcpp_verbose_abort()).
+#define _LIBCPP_ENABLE_ASSERTIONS 0
+
 #include "demangle/ItaniumDemangle.h"
 #include "__cxxabi_config.h"
 #include <cassert>
 #include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <functional>