string_constantt::to_array_expr must maintain the source location

The C front-end uses this method to create arrays from string literals.
The resulting expression must have a source location to ensure the
resulting GOTO instruction knows which source location to use.

Fixes: #6994

Author: tautschnig

regression/cbmc/String_Literal1/main.c

@@ -52,4 +52,6 @@ int main()
 
   // generic wide string, OS-dependent
   assert(sizeof(L""[0])==sizeof(wchar_t));
+
+  assert(0);
 }

regression/cbmc/String_Literal1/test.desc

@@ -1,8 +1,15 @@
 CORE
 main.c
-
-^EXIT=0$
+--trace
+^State \d+ file main.c function main line 20 thread 0$
+^State \d+ file main.c function main line 36 thread 0$
+^State \d+ file main.c function main line 44 thread 0$
+^\*\* 1 of \d+ failed \(\d+ iterations\)$
+^VERIFICATION FAILED$
+^EXIT=10$
 ^SIGNAL=0$
-^VERIFICATION SUCCESSFUL$
 --
 ^warning: ignoring
+^State \d+ (function main )?thread 0$
+--
+Each step in the trace must have a full source location, including line numbers.

src/util/string_constant.cpp

@@ -53,5 +53,8 @@ array_exprt string_constantt::to_array_expr() const
     *it = from_integer(ch, char_type);
   }
 
+  if(source_location().is_not_nil())
+    dest.add_source_location() = source_location();
+
   return dest;
 }