Add support for Mac SDK 15

Test: Manual
Change-Id: If54622940e731bef6c613989db4763344daafa06
diff --git a/cc/config/darwin_host.go b/cc/config/darwin_host.go
index 1783f49..716965a 100644
--- a/cc/config/darwin_host.go
+++ b/cc/config/darwin_host.go
@@ -54,6 +54,7 @@
 		"12",
 		"13",
 		"14",
+		"15",
 	}
 
 	darwinAvailableLibraries = append(