Make void_type() return empty_typet()

This is consistent with what is done in the Java front-end. Also use the same
trick to construct only a single, shared instance of it.

Author: tautschnig

src/util/c_types.cpp

@@ -250,9 +250,10 @@ reference_typet reference_type(const typet &subtype)
   return reference_typet(subtype, config.ansi_c.pointer_width);
 }
 
-typet void_type()
+empty_typet void_type()
 {
-  return empty_typet();
+  static const auto result = empty_typet();
+  return result;
 }
 
 std::string c_type_as_string(const irep_idt &c_type)

src/util/c_types.h

@@ -36,7 +36,7 @@ unsignedbv_typet size_type();
 signedbv_typet signed_size_type();
 signedbv_typet pointer_diff_type();
 pointer_typet pointer_type(const typet &);
-typet void_type();
+empty_typet void_type();
 
 // This is for Java and C++
 reference_typet reference_type(const typet &);