[c++17] P0135R1: Guaranteed copy elision.

[c++17] P0135R1: Guaranteed copy elision.

When an object of class type is initialized from a prvalue of the same type
(ignoring cv qualifications), use the prvalue to initialize the object directly
instead of inserting a redundant elidable call to a copy constructor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288866 91177308-0d34-0410-b5e6-96231b3b80d8