When replacements have the same offset, make replacements with smaller length...

When replacements have the same offset, make replacements with smaller length order first in the set.

Summary:
No behavioral change intended. The change makes iterating the replacements set more intuitive in Replacements class implementation. Previously, insertion is ordered before an deletion/replacement with the same offset, which is counter-intuitive for implementation, especially for a followup patch to support adding insertions around replacements.

With the current ordering, we only need to make `applyAllReplacements` iterate the replacements set reversely when applying them so that deletion/replacement is still applied before insertion with the same offset.

Reviewers: klimek, djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D24663

llvm-svn: 281819