Add external projects only CFlags option

Every time the toolchain adds a new warning, we had to suppress the
warning in every external projects that violates it, or disable it
globally -- which we end up doing most of the time since the first
option is way too time consuming.

Add a separate cflags option for external projects and move existing
-Wno-* flags there so that we can enforce better code quality for
internal projects.

TODO: Move more -Wno-* flags to ClangExtraExternalCflags

Test: m checkbuild
Change-Id: If3dee491616a1e7ba6223c2f522d100e10c5ee76

cc/compiler.go

@@ -392,6 +392,12 @@ func (compiler *baseCompiler) compilerFlags(ctx ModuleContext, flags Flags, deps
 			fmt.Sprintf("${config.%sGlobalCflags}", hod))
 	}
 
+	if flags.Clang {
+		if strings.HasPrefix(android.PathForModuleSrc(ctx).String(), "external/") {
+			flags.GlobalFlags = append([]string{"${config.ClangExternalCflags}"}, flags.GlobalFlags...)
+		}
+	}
+
 	if ctx.Device() {
 		if Bool(compiler.Properties.Rtti) {
 			flags.CppFlags = append(flags.CppFlags, "-frtti")