Add option to allow unknown warning options

Some build targets pulls cflags from build/soong but has its own choice
of compiler version. This causes errors for due to unknown warning
options. This allows them to suppress such checks.

Test: presubmit
Change-Id: Ia89508d6b92481fb1ed0c90f1fdb4d8e54566349

cc/config/global.go

@@ -375,6 +375,10 @@ func init() {
 			flags = append(flags, llvmNextExtraCommonGlobalCflags...)
 		}
 
+		if ctx.Config().IsEnvTrue("ALLOW_UNKNOWN_WARNING_OPTION") {
+			flags = append(flags, "-Wno-error=unknown-warning-option")
+		}
+
 		return strings.Join(flags, " ")
 	})