Improve our handling of the second step in a user-defined conversion

sequence. Previously, we weren't permitting the second step to call
copy constructors, which left user-defined conversion sequences
surprisingly broken.

Now, we perform overload resolution among all of the constructors, but
only accept the result if it makes the conversion a standard
conversion. Note that this behavior is different from both GCC and EDG
(which don't agree with each other, either); I've submitted a core
issue on the matter.

llvm-svn: 63450