C front-end: Do not wrongly propagate const-ness

This bug has been lingering for a longer time, but only started showing up with
the propagation fix in 2c16231b48.

1. Qualifiers (and const-ness in particular) are a property of the type, and not
an expression. Make sure all uses of ID_C_constant affect the type.
2. Qualifiers go with the type of the symbol being declared, and aren't a
property that applies across uses of the same type.

regression/ansi-c/const2/main.c

@@ -0,0 +1,36 @@
+#if TEST == 0
+const struct buffer *foo();
+
+typedef struct buffer
+{
+  int head;
+} buffer_t;
+#endif
+
+#if TEST == 1
+struct buffer *foo();
+
+typedef struct buffer
+{
+  int head;
+} buffer_t;
+#endif
+
+#if TEST == 2
+typedef struct buffer
+{
+  int head;
+} buffer_t;
+
+const struct buffer *foo();
+#endif
+
+static void clear(buffer_t *buf)
+{
+  buf->head = 0u;
+}
+
+int main()
+{
+  return 0;
+}

regression/ansi-c/const2/test0.desc

@@ -0,0 +1,7 @@
+CORE
+main.c
+-DTEST=0
+^EXIT=0$
+^SIGNAL=0$
+--
+^.*: .* is constant$

regression/ansi-c/const2/test1.desc

@@ -0,0 +1,7 @@
+CORE
+main.c
+-DTEST=1
+^EXIT=0$
+^SIGNAL=0$
+--
+^.*: .* is constant$

regression/ansi-c/const2/test2.desc

@@ -0,0 +1,7 @@
+CORE
+main.c
+-DTEST=2
+^EXIT=0$
+^SIGNAL=0$
+--
+^.*: .* is constant$

src/ansi-c/anonymous_member.cpp

@@ -29,10 +29,11 @@ static exprt make_member_expr(
   const typet &type=
     ns.follow(struct_union.type());
 
-  if(result.get_bool(ID_C_constant) ||
-     type.get_bool(ID_C_constant) ||
-     struct_union.type().get_bool(ID_C_constant))
-    result.set(ID_C_constant, true);
+  if(
+    type.get_bool(ID_C_constant) || struct_union.type().get_bool(ID_C_constant))
+  {
+    result.type().set(ID_C_constant, true);
+  }
 
   return result;
 }

src/ansi-c/c_typecast.cpp

@@ -555,12 +555,6 @@ void c_typecastt::implicit_typecast_followed(
 
       // check qualifiers
 
-      /*
-      if(src_type.subtype().get_bool(ID_C_constant) &&
-         !dest_type.subtype().get_bool(ID_C_constant))
-        warnings.push_back("disregarding const");
-      */
-
       if(src_type.subtype().get_bool(ID_C_volatile) &&
          !dest_type.subtype().get_bool(ID_C_volatile))
         warnings.push_back("disregarding volatile");

src/ansi-c/c_typecheck_expr.cpp

@@ -358,8 +358,8 @@ void c_typecheck_baset::typecheck_expr_main(exprt &expr)
       if(op.get_bool(ID_C_lvalue))
         complex_real_expr.set(ID_C_lvalue, true);
 
-      if(op.get_bool(ID_C_constant))
-        complex_real_expr.set(ID_C_constant, true);
+      if(op.type().get_bool(ID_C_constant))
+        complex_real_expr.type().set(ID_C_constant, true);
 
       expr.swap(complex_real_expr);
     }
@@ -396,8 +396,8 @@ void c_typecheck_baset::typecheck_expr_main(exprt &expr)
       if(op.get_bool(ID_C_lvalue))
         complex_imag_expr.set(ID_C_lvalue, true);
 
-      if(op.get_bool(ID_C_constant))
-        complex_imag_expr.set(ID_C_constant, true);
+      if(op.type().get_bool(ID_C_constant))
+        complex_imag_expr.type().set(ID_C_constant, true);
 
       expr.swap(complex_imag_expr);
     }
@@ -2853,7 +2853,7 @@ void c_typecheck_baset::typecheck_function_call_arguments(
       if(type.id()==ID_array)
       {
         typet dest_type=pointer_type(void_type());
-        dest_type.subtype().set(ID_C_constant, ID_1);
+        dest_type.subtype().set(ID_C_constant, true);
         implicit_typecast(op, dest_type);
       }
     }

src/ansi-c/c_typecheck_type.cpp

@@ -722,6 +722,16 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
 
   bool have_body=type.find(ID_components).is_not_nil();
 
+  c_qualifierst original_qualifiers(type);
+
+  // the type symbol, which may get re-used in other declarations, must not
+  // carry any qualifiers (other than transparent_union, which isn't really a
+  // qualifier)
+  c_qualifierst remove_qualifiers;
+  remove_qualifiers.is_transparent_union =
+    original_qualifiers.is_transparent_union;
+  remove_qualifiers.write(type);
+
   if(type.find(ID_tag).is_nil())
   {
     // Anonymous? Must come with body.
@@ -818,8 +828,6 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
     }
   }
 
-  c_qualifierst original_qualifiers(type);
-
   typet tag_type;
 
   if(type.id() == ID_union || type.id() == ID_incomplete_union)