[c++17] Refine resolution of constructor / conversion function disambiguation.

Given a choice between a constructor call and a conversion function in C++17,
we prefer the constructor for direct-initialization and the conversion function
for copy-initialization, matching the behavior in C++14 and before. The
guaranteed copy elision rules were not intended to change the meaning of such
code (other than by removing unnecessary copy constructor calls).

This tweak will be raised with CWG.

llvm-svn: 317066