Don't call android.PathForModuleSrc(ctx).String() to get ModuleDir

ctx.ModuleDir is much cheaper than android.PathForModuleSrc(ctx).String().

Test: builds
Change-Id: I81819088d9564b06a0336a59a45f2b110b0bd9d6
diff --git a/cc/builder.go b/cc/builder.go
index c8fa1e4..3f582fa 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -534,7 +534,7 @@
 		toolingCppflags += " ${config.NoOverride64GlobalCflags}"
 	}
 
-	modulePath := android.PathForModuleSrc(ctx).String()
+	modulePath := ctx.ModuleDir()
 	if android.IsThirdPartyPath(modulePath) {
 		cflags += " ${config.NoOverrideExternalGlobalCflags}"
 		toolingCflags += " ${config.NoOverrideExternalGlobalCflags}"