Implement basic support for converting constructors in user-defined

conversions.

Notes:
  - Overload resolution for converting constructors need to prohibit
    user-defined conversions (hence, the test isn't -verify safe yet).
  - We still use hacks for conversions from a class type to itself. 
    This will be the case until we start implicitly declaring the appropriate
    special member functions. (That's next on my list)

llvm-svn: 58513