[analyzer] Fix handling of labels in getLValueElement

In getLValueElement Base may represent the address of a label 
(as in the newly-added test case), in this case it's not a loc::MemRegionVal 
and Base.castAs<loc::MemRegionVal>() triggers an assert, this diff makes 
getLValueElement return UnknownVal instead.

Differential revision: https://reviews.llvm.org/D39174

llvm-svn: 316399