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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316399 91177308-0d34-0410-b5e6-96231b3b80d8