fix a few compilation warnings

- remove unused variables and class fields
 - add support for gcc 4.5 & clang's __builtin_unreachable
 - fix 2 bugs related to strict aliasing
 - remove a few unused function parameters

Signed-off-by: Nuno Lopes <t-nclaud@microsoft.com>

src/ast/ast.h

@@ -1193,7 +1193,6 @@ enum pattern_op_kind {
    heurisitic quantifier instantiation.
 */
 class pattern_decl_plugin : public decl_plugin {
-    sort * m_list;
 public:
     virtual decl_plugin * mk_fresh() { return alloc(pattern_decl_plugin); }