Fix String initialiser type when java.lang.String is a stub

Previously it was accidentally given type java.lang.Object, but we
didn't notice because we were less stringent about verifying type
equality (or, in transitioning from ID_symbol to ID_struct_tag for
Java types, we've become less generous about permitting a structural
match but name mismatch). Either way it was clearly wrong to assign
a wrong type like this.

Author: Owen Jones

jbmc/src/java_bytecode/java_string_literals.cpp

@@ -204,6 +204,7 @@ symbol_exprt get_or_create_string_literal_symbol(
     // Case where java.lang.String was stubbed, and so directly defines
     // @class_identifier
     new_symbol.value = jlo_init;
+    new_symbol.value.type() = string_type;
   }
 
   bool add_failed = symbol_table.add(new_symbol);