Remove the use of pair inside the tuple in concat_iterator.
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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@340567 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm