Adds LLVM hardening flag for cc_toolchain libc++ for LLVM >= 17 (#3052)

LLVM 17 replaces ENABLE_ASSERTIONS with ENABLE_HARDEDNING. LLVM 18 is
temporarily warning that ENABLE_ASSERTIONS is deprecated. Since we treat
warnings as errors, this causes build failures for LLVM 18.

This commit can probably be safely removed/undone once LLVM hardening
removes the check
[here](https://github.com/llvm/llvmproject/blob/667602793bef72e64367ac46cbaf96e3afaea22e/libcxx/include/__config#L211).

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

bazel/cc_toolchains/clang_cc_toolchain_config.bzl

@@ -511,10 +511,13 @@ def _impl(ctx):
 
     # With clang 14 and lower, we expect it to be built with libc++ debug
     # support. In later LLVM versions, we expect the assertions define to work.
+    # clang 17 deprecates LIBCPP_ENABLE_ASSERTIONS in favor of HARDENED_MODE.
     if clang_version and clang_version <= 14:
         libcpp_debug_flags = ["-D_LIBCPP_DEBUG=1"]
-    else:
+    elif clang_version and clang_version <= 16:
         libcpp_debug_flags = ["-D_LIBCPP_ENABLE_ASSERTIONS=1"]
+    else:
+        libcpp_debug_flags = ["-D_LIBCPP_ENABLE_HARDENED_MODE=1"]
 
     linux_flags_feature = feature(
         name = "linux_flags",