Wshift-sign-overflow

See:
http://stackoverflow.com/questions/26331035/why-was-1-31-changed-to-be-implementation-defined-in-c14

And Howard Hinnant's explanation:
http://stackoverflow.com/questions/19593938/is-left-shifting-a-negative-integer-undefined-behavior-in-c11#comment29091986_19593938

src/ast/ast.h

@@ -522,7 +522,7 @@ public:
 /**
    The ids of expressions and declarations are in different ranges. 
 */
-const unsigned c_first_decl_id = (1 << 31);
+const unsigned c_first_decl_id = (1u << 31);
 
 /**
    \brief Superclass for function declarations and sorts.