provided_by_macos_depends_on_allowlist: add `llvm`

I've seen more than a handful of PRs that fail the audit requesting that
a formula do `uses_from_macos` instead of `depends_on` for `llvm`, but
the audit was wrong in every one of those instances. The `llvm` formula
includes much more than what comes with macOS.

Closes #84391.

Signed-off-by: rui <rui@chenrui.dev>
Signed-off-by: BrewTestBot <1589480+BrewTestBot@users.noreply.github.com>

audit_exceptions/provided_by_macos_depends_on_allowlist.json

@@ -2,6 +2,7 @@
   "apr",
   "apr-util",
   "libressl",
+  "llvm",
   "openblas",
   "openssl@1.1"
 ]