[llvm] Separate out reverse iteration flag into its own header

Summary: This will ease out adding reverse iteration flags to other containers by simply including the header.

Reviewers: mehdi_amini, dexonsmith, davide, dblaikie

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35042

llvm-svn: 307297