Default to pattern initialization for uninitialized variables.

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: I9705ca3b724208647f0eab0a704f6f360206d482

cc/config/global.go

@@ -165,6 +165,9 @@ func init() {
 			flags = append(flags, "-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang")
 		} else if ctx.Config().IsEnvTrue("AUTO_PATTERN_INITIALIZE") {
 			flags = append(flags, "-ftrivial-auto-var-init=pattern")
+		} else {
+			// Default to pattern initialization.
+			flags = append(flags, "-ftrivial-auto-var-init=pattern")
 		}
 
 		return strings.Join(flags, " ")