[analyzer] Use 'MemRegion::printPretty()' instead of assuming the region is a VarRegion.

Fixes PR15358 and <rdar://problem/13295437>.

Along the way, shorten path diagnostics that say "Variable 'x'" to just
be "'x'".  By the context, it is obvious that we have a variable,
and so this just consumes text space.

llvm-svn: 176115