GRTransferFuncs had the conflated role of both constructing SVals (symbolic
expressions) as well as handling checker-specific logic. Now SValuator has the
role of constructing SVals from expressions and GRTransferFuncs just handles
checker-specific logic. The motivation is by separating these two concepts we
will be able to much more easily create richer constraint-generating logic
without coupling it to the main checker transfer function logic.

We now have one implementation of SValuator: SimpleSValuator.

SimpleSValuator is essentially the SVal-related logic that was in GRSimpleVals
(which is removed in this patch). This includes the logic for EvalBinOp,
EvalCast, etc. Because SValuator has a narrower role than the old
GRTransferFuncs, the interfaces are much simpler, and so is the implementation
of SimpleSValuator compared to GRSimpleVals. I also did a line-by-line review of
SVal-related logic in GRSimpleVals and cleaned it up while moving it over to
SimpleSValuator.

As a consequence of removing GRSimpleVals, there is no longer a
'-checker-simple' option. The '-checker-cfref' did everything that option did
but also ran the retain/release checker. Of course a user may not always wish to
run the retain/release checker, nor do we wish core analysis logic buried in the
checker-specific logic. The next step is to refactor the logic in CFRefCount.cpp
to separate out these pieces into the core analysis engine.

llvm-svn: 74229