Fix spurious warning about cast of unrelated types

[DutChen18]

Fixed a spurious warning about cast of unrelated types, as seen in this example program (Commenting out the namespace declaration on line 9 causes the warning not to appear):

#include <cheerp/types.h>

class CString {
public:
        [[cheerp::genericjs]]
        explicit operator client::String*() const;
};

namespace [[cheerp::genericjs]] client {}

[[cheerp::genericjs]]
client::Object* foo() {
        CString string;
        return static_cast<client::String*>(string);
}

Produces the warning:

test.cpp:14:16: warning: Cheerp: Using values cast to unrelated types is undefined behaviour unless the destination type is the actual type of the value [-Wcheerp-unsafe]
        return static_cast<client::String*>(string);
               ^

I believe the issue occurs because the client::String type spelled in the declaration of operator client::String* on line 6 is not the exact same type as the one in the static_cast on line 14. However, they do have the same canonical type.

I looked at a few different functions in ASTContext to compare the types in a better way. The one that seems to make the most sense is hasSameType, which internally calls getCanonicalType on both arguments and just does an == comparison on the resulting pointers.

[yuri91]

hasSameType looks like the correct choice. i saw it used in a bunch of places too.