cope with the case where a user-defined conversion is actually a copy
construction, and therefore can be compared against other standard
conversion sequences. While I called this a hack before, now I'm
convinced that it's the right way to go.

Compare overloads based on derived-to-base conversions that invoke
copy constructors. 

Suppress user-defined conversions when attempting to call a
user-defined conversion.

llvm-svn: 58629