Add a helper getter method for common settings

rules/common_settings.bzl

@@ -98,3 +98,7 @@ string_setting = rule(
     },
     doc = "A string-typed build setting that cannot be set on the command line",
 )
+
+# Extracts the value from an attribute that returns BuildSettingInfo
+def get_build_setting_value(build_setting_attr):
+    return build_setting_attr[BuildSettingInfo].value