Move ID_nondet_padding inside the scope of class

As requested in code review.

Author: thomasspriggs

src/solvers/Makefile

@@ -211,6 +211,7 @@ SRC = $(BOOLEFORCE_SRC) \
       smt2_incremental/smt2_incremental_decision_procedure.cpp \
       smt2_incremental/encoding/struct_encoding.cpp \
       smt2_incremental/encoding/enum_encoding.cpp \
+      smt2_incremental/encoding/nondet_padding.cpp \
       smt2_incremental/theories/smt_array_theory.cpp \
       smt2_incremental/theories/smt_bit_vector_theory.cpp \
       smt2_incremental/theories/smt_core_theory.cpp \

src/solvers/smt2_incremental/encoding/nondet_padding.cpp

@@ -0,0 +1,5 @@
+// Author: Diffblue Ltd.
+
+#include "nondet_padding.h"
+
+const irep_idt nondet_padding_exprt::ID_nondet_padding = "nondet_padding";

src/solvers/smt2_incremental/encoding/nondet_padding.h

@@ -13,11 +13,11 @@
 class nondet_padding_exprt;
 void validate_expr(const nondet_padding_exprt &padding);
 
-const irep_idt ID_nondet_padding = "nondet_padding";
-
 class nondet_padding_exprt : public expr_protectedt
 {
 public:
+  static const irep_idt ID_nondet_padding;
+
   explicit nondet_padding_exprt(typet type)
     : expr_protectedt{ID_nondet_padding, std::move(type)}
   {
@@ -28,7 +28,7 @@ class nondet_padding_exprt : public expr_protectedt
 template <>
 inline bool can_cast_expr<nondet_padding_exprt>(const exprt &base)
 {
-  return base.id() == ID_nondet_padding;
+  return base.id() == nondet_padding_exprt::ID_nondet_padding;
 }
 
 inline void validate_expr(const nondet_padding_exprt &padding)