[analyzer] add the notion of an "interesting" lvalue expression for ExplodedNode pruning.

r175988 modified the ExplodedGraph trimming algorithm to retain all
nodes for "lvalue" expressions.  This patch refines that notion to
only "interesting" expressions that would be used for diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176010 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed