reverses common and external Clang flags

Flags for external projects are specialisations of the common flags, and
need to follow the common flags, not precede them.

Bug: 181177782
Test: None

Change-Id: I19c8c2a3539573e9b2f2d9e3e1c898fa09570663
diff --git a/cc/compiler.go b/cc/compiler.go
index e96295c..2e71922 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -442,7 +442,7 @@
 		fmt.Sprintf("${config.%sClangGlobalCflags}", hod))
 
 	if isThirdParty(modulePath) {
-		flags.Global.CommonFlags = append([]string{"${config.ClangExternalCflags}"}, flags.Global.CommonFlags...)
+		flags.Global.CommonFlags = append(flags.Global.CommonFlags, "${config.ClangExternalCflags}")
 	}
 
 	if tc.Bionic() {