Resolve a defect in C++17 copy omission.

When selecting constructors for initializing an object of type T from a single
expression of class type U, also consider conversion functions of U that
convert to T (rather than modeling such conversions as calling a conversion
function and then calling a constructor).

This approach is proposed as the resolution for the defect, and is also already
implemented by GCC.

llvm-svn: 314231