Remove ProgramState::getSymVal(). It was being misused by Checkers,

Remove ProgramState::getSymVal().  It was being misused by Checkers,
with at least one subtle bug in MacOSXKeyChainAPIChecker where the
calling the method was a substitute for assuming a symbolic value
was null (which is not the case).

We still keep ConstraintManager::getSymVal(), but we use that as
an optimization in SValBuilder and ProgramState::getSVal() to
constant-fold SVals.  This is only if the ConstraintManager can
provide us with that information, which is no longer a requirement.
As part of this, introduce a default implementation of
ConstraintManager::getSymVal() which returns null.

For Checkers, introduce ConstraintManager::isNull(), which queries
the state to see if the symbolic value is constrained to be a null
value.  It does this without assuming it has been implicitly constant
folded.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163428 91177308-0d34-0410-b5e6-96231b3b80d8