C front-end: hide our use of single-bit bool within sizeof

In C, Boolean operations are of type int. Rejecting sizeof expressions
of Boolean operations breaks building the Linux kernel.

Author: tautschnig

regression/ansi-c/sizeof4/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 
 ^EXIT=0$

src/ansi-c/c_typecheck_expr.cpp

@@ -931,7 +931,21 @@ void c_typecheck_baset::typecheck_expr_sizeof(exprt &expr)
   }
   else
   {
-    type.swap(to_unary_expr(expr).op().type());
+    const exprt &op = to_unary_expr(as_const(expr)).op();
+    // This is one of the few places where it's detectable
+    // that we are using "bool" for boolean operators instead
+    // of "int". We convert for this reason.
+    if(
+      op.type().id() == ID_bool &&
+      (op.id() == ID_not || op.id() == ID_and || op.id() == ID_or ||
+       op.id() == ID_equal || op.id() == ID_notequal || op.id() == ID_lt ||
+       op.id() == ID_le || op.id() == ID_gt || op.id() == ID_ge ||
+       op.id() == ID_if))
+    {
+      type = signed_int_type();
+    }
+    else
+      type = op.type();
   }
 
   exprt new_expr;