[analyzer] Only adjust the type of 'this' when we devirtualize a method call.

With reinterpret_cast, we can get completely unrelated types in a region
hierarchy together; this was resulting in CXXBaseObjectRegions being layered
directly on an (untyped) SymbolicRegion, whose symbol was from a completely
different type hierarchy. This was what was causing the internal buildbot to
fail.

Reverts r161911, which merely masked the problem.

llvm-svn: 161960