Permit constructing parsert with a message handler

This is to provide a path to remove the long-deprecated messaget()
constructor. Uses of this new constructor will be added in separate
commits.

src/util/parser.h

@@ -12,6 +12,7 @@ Author: Daniel Kroening, [email protected]
 #ifndef CPROVER_UTIL_PARSER_H
 #define CPROVER_UTIL_PARSER_H
 
+#include "deprecate.h"
 #include "expr.h"
 #include "message.h"
 
@@ -39,7 +40,13 @@ class parsert
     last_line.clear();
   }
 
+  DEPRECATED(SINCE(2023, 12, 20, "use parsert(message_handler) instead"))
   parsert():in(nullptr) { clear(); }
+  explicit parsert(message_handlert &message_handler)
+    : in(nullptr), log(message_handler)
+  {
+    clear();
+  }
   virtual ~parsert() { }
 
   // The following are for the benefit of the scanner