build: recent macOS updates clang (#579)

Not sure what you want to do with this, but on Sonoma, clang 15.0.0 is
what is installed.

I'm not currently able to build on my OSX machine, seemingly because I
can't get bazel and my nix libtool package to cooperate.

BUILD.bazel

@@ -76,8 +76,8 @@ config_setting(
 # Version flag for clang.
 string_flag(
     name = "clang_version",
-    # macOS uses `clang-14.0.3` by default.
-    build_setting_default = "14.0.3",
+    # macOS uses `clang-15.0.0` by default.
+    build_setting_default = "15.0.0",
     visibility = ["//visibility:public"],
 )