Implement P0033R1 - Re-enabling shared_from_this

Summary: See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0033r1.html

Reviewers: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19254

llvm-svn: 271449