Use the object's package to mangle method names, rather than the receiver's package

If we use the receiver's package, we can end up with identical manglings
for different functions. Consider:

package p
type U struct{}
func (U) f()

package q
import "p"
type T struct { p.U }
func (T) f()

The method set of *T has two synthetic methods named (*T).f(); one forwards to
(T).f(), and the other to (U).f(). Previously, we were distinguishing them
by the receiver's package, and in this case because both methods have the
same receiver, they received the same name.

The methods are correctly distinguished by the package owning the identifier
"f", which is available via f.Object().Pkg().

Differential Revision: http://reviews.llvm.org/D6673

llvm-svn: 224357