Implement std::experimental::propagate_const from LFTS v2

Summary:
An implementation of std::experimental::propagate_const from Library Fundamentals Technical Specification v2.

No tests are provided for disallowed types like fancy pointers or function pointers as no code was written to handle these.

Reviewers: EricWF, mclow.lists

Differential Revision: http://reviews.llvm.org/D12486

llvm-svn: 273122