Improve our handling of user-defined conversions when computing

implicit conversion sequences. In particular, model the "standard
conversion" from a class to its own type (or a base type) directly as
a standard conversion in the normal path *without* trying to determine
if there is a valid copy constructor. This appears to match the intent
of C++ [over.best.ics]p6 and more closely matches GCC and EDG.

As part of this, model non-lvalue reference initialization via
user-defined conversion in overloading the same way we handle it in
InitializationSequence, separating the "general user-defined
conversion" and "conversion to compatible class type" cases.

The churn in the overload-call-copycon.cpp test case is because the
test case was originally wrong; it assumed that we should do more
checking for copy constructors that we actually should, which affected
overload resolution.

Fixes PR7055. Bootstrapped okay.

llvm-svn: 110773