[TableGen] Allow TokenTy in intrinsic signatures

Summary:
Add the necessary plumbing so that llvm_token_ty can be used as an
argument/return type in intrinsic definitions and correspondingly require
TokenTy in function types.  TokenTy is an opaque type that has no target
lowering, but can be used in machine-independent intrinsics.  It is
required for the upcoming llvm.eh.padparam intrinsic.

Reviewers: majnemer, rnk

Subscribers: stoklund, llvm-commits

Differential Revision: http://reviews.llvm.org/D12532

llvm-svn: 246651