namespacet::follow: also follow enum tags

`follow` is a catch-all for tagged (and untagged) types that should do
all variants of `follow_tag`. Fixes an unbounded recursion when using
nondet-volatile on programs that use enums.

regression/goto-instrument/nondet-volatile-01/test.c

@@ -1,5 +1,15 @@
 #include <assert.h>
 
+enum E
+{
+  A,
+  B
+};
+
+int main()
+{
+}
+
 void main()
 {
   int a[2] = {0};
@@ -8,4 +18,8 @@ void main()
   a[i] = 1;
 
   assert(a[1] == 0); // should fail
+
+  // make sure the use of enum (tags) does not cause infinite recursion
+  enum A a = A;
+  a = a;
 }

src/util/namespace.cpp

@@ -54,6 +54,9 @@ const typet &namespace_baset::follow(const typet &src) const
   if(src.id() == ID_struct_tag)
     return follow_tag(to_struct_tag_type(src));
 
+  if(src.id() == ID_c_enum_tag)
+    return follow_tag(to_c_enum_tag_type(src));
+
   return src;
 }