Remove the use of pair inside the tuple in concat_iterator.

Summary:
Remove the use of pair inside the tuple in concat_iterator, and create separate begins and ends tuples instead.
This fixes the failure for llvm <= 3.7 and libstd++ that broke the hexagon build.

Reviewers: timshen

Subscribers: sanjoy, jlebar, dexonsmith, kparzysz, llvm-commits

Differential Revision: https://reviews.llvm.org/D51067

llvm-svn: 340567