[SelectionDAGBuilder] Adds support for landingpads of token type

Summary: This patch adds a check in visitLandingPad to see if landingpad's result type is token type. If so, do not create DAG nodes for its exception pointer and selector value. This patch enables the back end to handle landingpads of token type.

Reviewers: JosephTremoulet, majnemer, rnk

Subscribers: sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D15405

llvm-svn: 255749