Default to pattern initialization for uninitialized variables (try 2).

Pattern initialization helps us make C++ safer, while not altering the
semantics/usage of C++ (as zero init does).

Bug: 131390872
Test: Local testing. Parts of CTS.
Change-Id: Ic4af9260a48c10cbd70315fa56d6b01c5ca61768

cc/config/global.go

@@ -168,6 +168,9 @@ func init() {
 			flags = append(flags, "-ftrivial-auto-var-init=pattern")
 		} else if ctx.Config().IsEnvTrue("AUTO_UNINITIALIZE") {
 			flags = append(flags, "-ftrivial-auto-var-init=uninitialized")
+		} else {
+			// Default to pattern initialization.
+			flags = append(flags, "-ftrivial-auto-var-init=pattern")
 		}
 
 		return strings.Join(flags, " ")