[libc++][hardening] Introduce assertion semantics (#148268)

Assertion semantics closely mimic C++26 Contracts evaluation semantics.
This brings our implementation closer in line with C++26 Library
Hardening (one particular benefit is that using the `observe` semantic
makes adopting hardening easier for projects).

NOKEYCHECK=True
GitOrigin-RevId: 7345508c6febc57eaac985ef2fb14beabdc4d461

src/demangle/DemangleConfig.h

diff --git a/src/demangle/DemangleConfig.h b/src/demangle/DemangleConfig.h
index 06fd223..7904e9d 100644
--- a/src/demangle/DemangleConfig.h
+++ b/src/demangle/DemangleConfig.h
@@ -19,6 +19,14 @@
 #include "../abort_message.h"
 #endif
 
+#ifndef _LIBCPP_LOG_HARDENING_FAILURE
+// Libc++abi does not have any functionality to log and continue, so we drop
+// error messages when we build the demangler with `observe` assertion semantic.
+// Once the layering with libc++ is improved, this could use the libc++
+// functionality to log hardening failures.
+#define _LIBCPP_LOG_HARDENING_FAILURE(message) ((void)0)
+#endif
+
 #include <version>
 
 #ifdef _MSC_VER