Improve our handling of user-defined conversions as part of overload

resolution. There are two sources of problems involving user-defined
conversions that this change eliminates, along with providing simpler
interfaces for checking implicit conversions:

  - It eliminates a case of infinite recursion found in Boost.

  - It eliminates the search for the constructor needed to copy a temporary
    generated by an implicit conversion from overload
    resolution. Overload resolution assumes that, if it gets a value
    of the parameter's class type (or a derived class thereof), there
    is a way to copy if... even if there isn't. We now model this
    properly.

llvm-svn: 101680