Fix gnuv3_pass_by_reference to lookup copy c-tors with qualified args.

Fix gnuv3_pass_by_reference to lookup copy c-tors with qualified args.

Before this, a copy constructor declared as in the following snippet was
not being treated as a copy constructor.

class A
{
public:
  A (A &); // OK.
  A (const A &); // Not being treated as a copy constructor because of the
                 // 'const' qualifier.
};

gdb/ChangeLog:

	PR c++/13403
	PR c++/15154
	* gnu-v3-abi.c (gnuv3_pass_by_reference): Lookup copy constructors
	with qualified args.