[libc++] Add a link to __builtin_verbose_trap from the hardening docs (llvm#126930)

Author: ldionne

libcxx/docs/Hardening.rst

@@ -248,14 +248,12 @@ Hardening assertion failure
 ===========================
 
 In production modes (``fast`` and ``extensive``), a hardening assertion failure
-immediately ``_traps <https://llvm.org/docs/LangRef.html#llvm-trap-intrinsic>``
+immediately ``_traps <https://clang.llvm.org/docs/LanguageExtensions.html#builtin-verbose-trap>``
 the program. This is the safest approach that also minimizes the code size
 penalty as the failure handler maps to a single instruction. The downside is
 that the failure provides no additional details other than the stack trace
 (which might also be affected by optimizations).
 
-TODO(hardening): describe ``__builtin_verbose_trap`` once we can use it.
-
 In the ``debug`` mode, an assertion failure terminates the program in an
 unspecified manner and also outputs the associated error message to the error
 output. This is less secure and increases the size of the binary (among other