Begin major changes to EvalXXX methods in GRTransferFuncs.  Currently some of the methods only return an RVal; we want them to be able to create an arbitrary number of states.

llvm-svn: 53739

current store implementation is now encapsulated by BasicStore.

These changes prompted some long due constification of ValueState. Much of the
diffs in this patch include adding "const" qualifiers.

llvm-svn: 53423

represent possible stores to "Unknown."

llvm-svn: 49811

llvm-svn: 49805