Rename SVal::isType functions to SVal::isKind

Post-commit CR feedback from Jordan Rose regarding r175594.
Use op-> directly rather than via Optional<T>::getPointer.

Post-commit CR feedback from Jordan Rose regarding r175594.
Remove redundant Optional type in favor of llvm::Optional
Include llvm::Optional in clang/Basic/LLVM.h

Post-commit CR feedback from Jordan Rose regarding r175594.