Use std::string for ConstantExpression::*value() methods.

Bug: 31633795 memory leaks are partly fixed

Test: `mma`

Change-Id: Ia9275b86ca6d7445f4832994b5a70a2e93993eb1
diff --git a/EnumType.h b/EnumType.h
index 58c79cf..39051bb 100644
--- a/EnumType.h
+++ b/EnumType.h
@@ -89,10 +89,10 @@
     EnumValue(const char *name, const ConstantExpression *value = nullptr);
 
     std::string name() const;
-    const char *value() const;
-    const char *cppValue(ScalarType::Kind castKind) const;
-    const char *javaValue(ScalarType::Kind castKind) const;
-    const char *comment() const;
+    std::string value() const;
+    std::string cppValue(ScalarType::Kind castKind) const;
+    std::string javaValue(ScalarType::Kind castKind) const;
+    std::string comment() const;
 
 private:
     std::string mName;