fallback formatter: don't print #source_location=""

This makes default-formatted expressions less verbose.

Author: kroening

regression/cbmc/double_deref/double_deref_with_pointer_arithmetic.desc

@@ -1,7 +1,7 @@
 CORE
 double_deref_with_pointer_arithmetic.c
 --show-vcc
-^\{-[0-9]+\} derefd_pointer::derefd_pointer!0#1 = \{ symex_dynamic::dynamic_object1#3\[\[0\]\], symex_dynamic::dynamic_object1#3\[\[1\]\] \}\[cast\(mod #source_location=""\(main::argc!0@1#1, 2\), signedbv\[64\]\)\]
+^\{-[0-9]+\} derefd_pointer::derefd_pointer!0#1 = \{ symex_dynamic::dynamic_object1#3\[\[0\]\], symex_dynamic::dynamic_object1#3\[\[1\]\] \}\[cast\(mod\(main::argc!0@1#1, 2\), signedbv\[64\]\)\]
 ^\{1\} \(derefd_pointer::derefd_pointer!0#1 = address_of\(symex_dynamic::dynamic_object[23]\) \? main::argc!0@1#1 = 2 : \(derefd_pointer::derefd_pointer!0#1 = address_of\(symex_dynamic::dynamic_object[23]\) \? main::argc!0@1#1 = 1 : symex::invalid_object!0#0 = main::argc!0@1#1\)\)
 ^EXIT=0$
 ^SIGNAL=0$

src/util/format_expr.cpp

@@ -223,7 +223,7 @@ std::ostream &fallback_format_rec(std::ostream &os, const exprt &expr)
   os << expr.id();
 
   for(const auto &s : expr.get_named_sub())
-    if(s.first != ID_type)
+    if(s.first != ID_type && s.first != ID_C_source_location)
       os << ' ' << s.first << "=\"" << s.second.id() << '"';
 
   if(expr.has_operands())