Implement C++11 [over.match.copy]p1b2, which allows the use of

explicit conversion functions to initialize the argument to a
copy/move constructor that itself is the subject of direct
initialization. Since we don't have that much context in overload
resolution, we end up threading more flags :(.

Fixes <rdar://problem/10903741> / PR10456. 

llvm-svn: 151409