[RegionInfo] Fix dangling references created by moving RegionInfo objects

Summary: Region objects capture the address of the creating RegionInfo instance. Because the RegionInfo class is movable, moving a RegionInfo object creates dangling references. This patch fixes these references by walking the Regions post-move, and updating references to the new parent.

Reviewers: Meinersbur, grosser

Reviewed By: Meinersbur, grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31719

llvm-svn: 301175