Refactored ExprDeclBitVector into two classes:

 DeclBitVector
 ExprDeclBitVector (which subclasses the former)

DeclBitVector is for analyses that just want to track bitvector state
for declarations.

ExprDeclBitVector is for analyses that want to track bitvector state
for both both declarations and CFGBlock-level expressions.

llvm-svn: 42445