compilers: cpp: wire up stdlib assertions

None of the options set here affect ABI and are intended for detecting constraint
violations.

For GCC, we simply need to set -D_GLIBCXX_ASSERTIONS.

For Clang, the situation is far more complicated:
* LLVM 18 uses a 'hardened mode' (https://libcxx.llvm.org/Hardening.html).
  There are several levels of severity available here. I've chosen
  _LIBCPP_HARDENING_MODE_EXTENSIVE as the strongest-but-one. The strongest
  one (_DEBUG) doesn't affect ABI still but is reserved for stldebug.

* LLVM 15 uses a similar approach to libstdc++ called '_LIBCPP_ENABLE_ASSERTIONS'

Note that LLVM 17 while in development had fully deprecated _LIBCPP_ENABLE_ASSERTIONS
in favour of hardened, but changed its mind last-minute: https://discourse.llvm.org/t/rfc-hardening-in-libc/73925/4.

Signed-off-by: Sam James <[email protected]>

mesonbuild/compilers/cpp.py

@@ -647,6 +647,7 @@ def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]
             args.append('-fno-exceptions')
         if not options[key.evolve('rtti')].value:
             args.append('-fno-rtti')
+
         if options[key.evolve('debugstl')].value:
             args.append('-D_GLIBCXX_DEBUG=1')
         return args