compilers: cpp: relax assertion level for libc++

Followup to 90098473d5.

I changed my mind on this a few times. libcxx's documentation describes [0]
the hardening modes as:
"""
1) Unchecked mode/none, which disables all hardening checks.
2) Fast mode, which contains a set of security-critical checks that can be done
   with relatively little overhead in constant time and are intended to be used
   in production. We recommend most projects adopt this.
3) Extensive mode, which contains all the checks from fast mode and some additional
   checks for undefined behavior that incur relatively little overhead but aren’t
   security-critical. Production builds requiring a broader set of checks than fast
   mode should consider enabling extensive mode. The additional rigour impacts performance
   more than fast mode: we recommend benchmarking to determine if that is acceptable
   for your program.
4) Debug mode, which enables all the available checks in the library, including
   internal assertions, some of which might be very expensive. This mode is
   intended to be used for testing, not in production.
"""

We chose 3) before because it felt like a better fit for what we're trying
to do and the most equivalent option to libstdc++'s _GLIBCXX_ASSERTIONS, but on
reflection, maybe we're better off picking a default with less overhead and
more importantly guarantees constant time checks.

[0] https://libcxx.llvm.org/Hardening.html#using-hardening-modes

Bug: https://github.com/mesonbuild/meson/issues/12962
Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

mesonbuild/compilers/cpp.py

@@ -315,7 +315,7 @@ class ClangCPPCompiler(_StdCPPLibMixin, ClangCompiler, CPPCompiler):
         # Clang supports both libstdc++ and libc++
         args.append('-D_GLIBCXX_ASSERTIONS=1')
         if version_compare(self.version, '>=18'):
-            args.append('-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE')
+            args.append('-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST')
         elif version_compare(self.version, '>=15'):
             args.append('-D_LIBCPP_ENABLE_ASSERTIONS=1')