Enable -Wno-xor-as-pow for external code.

Bug: 145211022
Test: presubmit
Change-Id: I025842df61cfd3657b21159fe4c05da857043a4c
diff --git a/cc/config/global.go b/cc/config/global.go
index 1a358d1..a4e2975 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -278,8 +278,6 @@
 		// http://b/145211477
 		"-Wno-pointer-compare",
 		// http://b/145211022
-		"-Wno-xor-used-as-pow",
-		// http://b/145211022
 		"-Wno-final-dtor-non-final-class",
 
 		// http://b/165945989