goto-instrument: const quantifier

[stahlbauer]

@tautschnig goto-instrument falsely emits const quantifiers for some typedef:
typedef const unsigned int uint32_t; instead of typedef unsigned int uint32_t;.

The problem might be caused by using the following operator (and none of the .set(ID_C_constant, true); causes it): c_qualifierst &operator+=(const c_qualifierst &b) in c_qualifiers.h

Test case for reproducing the problem: typedef-const.zip