[analyzer] FindLastStoreBRVisitor was not actually finding stores.

The visitor walks back through the ExplodedGraph as expected, but
it wasn't actually keeping track of when a value was assigned. This
meant that it only worked when the value was assigned when the variable
was defined.

Tests in the next commit (dependent on another change).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161276 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed