[libc++] Assume newer LLVM versions for not-yet-released macOS versions

This makes the test suite forward-compatible with future versions of macOS.
Previously, the Lit features were built in a way that they would assume
that any newer macOS version doesn't contain any version of LLVM, which
doesn't make sense.

Author: ldionne

libcxx/utils/libcxx/test/features.py

@@ -587,7 +587,7 @@ def check_gdb(cfg):
     Feature(
         name="_target-has-llvm-17",
         when=lambda cfg: BooleanExpression.evaluate(
-            "target={{.+}}-apple-macosx{{14.[4-9](.0)?}}",
+            "target={{.+}}-apple-macosx{{14.[4-9](.0)?}} || target={{.+}}-apple-macosx{{1[5-9]([.].+)?}}",
             cfg.available_features,
         ),
     ),