Add tag to synthetic type made for String.format()

Having this type without a tag made it very hard to identify,
e.g. if we want to treat it differently from types which
are not synthetic (i.e. which come directly from the source
code). Compare with refined_string_typet, which has the tag
CPROVER_PREFIX "refined_string_type".

Author: Owen Jones

jbmc/src/java_bytecode/java_string_library_preprocess.cpp

@@ -1459,6 +1459,7 @@ code_blockt java_string_library_preprocesst::make_string_format_code(
   // a string, an integer, a floating point, a character, a boolean,
   // an object of which we take the hash code, or a date/time
   struct_typet structured_type;
+  structured_type.set_tag(CPROVER_PREFIX "string_formatter_variant");
   structured_type.components().emplace_back("string_expr", refined_string_type);
   structured_type.components().emplace_back(ID_int, java_int_type());
   structured_type.components().emplace_back(ID_float, java_float_type());