[RegionInfo] Add debug-time region viewer functions

Summary:
Analogously to Function::viewCFG(), RegionInfo::view() and RegionInfo::viewOnly() are meant to be called in debugging sessions. They open a viewer to show how RegionInfo currently understands the region hierarchy.

The functions viewRegion(Function*) and viewRegionOnly(Function*) invoke a fresh region analysis of the function in contrast to viewRegion(RegionInfo*) and viewRegionOnly(RegionInfo*) which show the current analysis result.

Reviewers: grosser

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11875

llvm-svn: 244444