[libcxxabi] allow downstreams to override _LIBCPP_VERBOSE_ABORT

[nickdesaulniers]

Summary

@asbirlea reports that Google does this for sanitizer analysis
downstream. Looks like a few #defines are added into a header which is
then injected into the build.

[nickdesaulniers]

@vitalybuka does it make more sense to do this or revert cl/507604661?

[nickdesaulniers]

• move abort_message.h include into guard

[vitalybuka]

This is documented feature llvm-project/libcxx/docs/UsingLibcxx.rst and cl/507604661 suppose to used. Before the CL is deprecated.

So this patch is LGTM.

libcxxabi/src/demangle/DemangleConfig.h

 //===--- DemangleConfig.h --------------------------------------*- C++ -*-===//
 //
 // 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
 //
 // This file is contains a subset of macros copied from
 // llvm/include/llvm/Demangle/DemangleConfig.h
 //===----------------------------------------------------------------------===//
 
 #ifndef LIBCXXABI_DEMANGLE_DEMANGLE_CONFIG_H
 #define LIBCXXABI_DEMANGLE_DEMANGLE_CONFIG_H
 
-// Must be defined before pulling in headers from libc++.
+// Must be defined before pulling in headers from libc++. Allow downstream
+// build systems to override this value.
 // https://libcxx.llvm.org/UsingLibcxx.html#enabling-the-safe-libc-mode
+#ifndef _LIBCPP_VERBOSE_ABORT
 #define _LIBCPP_VERBOSE_ABORT(...) abort_message(__VA_ARGS__)
-
 #include "../abort_message.h"
+#endif
+
 #include <ciso646>
 
 #ifdef _MSC_VER
 // snprintf is implemented in VS 2015
 #if _MSC_VER < 1900
 #define snprintf _snprintf_s
 #endif
 #endif