When performing copy initialization (= "implicit conversion", here) to
a class type from itself or a derived class thereof, enumerate
constructors and permit user-defined conversions to the arguments of
those constructors. This fixes the wacky implicit conversion sequence
used in std::auto_ptr's lame emulation of move semantics.

llvm-svn: 88670
GitOrigin-RevId: 379d84b7ed24980a3dbd3d60e4ab2bacce290c0c