[Acc] Do not statically dispatch into IslNodeBuilder's createFor

Summary:
When GPUNodeBuilder creates loops inside the kernel, it dispatches to
IslNodeBuilder. This however is surprisingly dangerous, since it accesses the
AST Node's user through the wrong type. This patch fixes this problem by
overriding createFor correctly.

This fixes PR35010.

Reviewers: grosser, bollu, Meinersbur

Reviewed By: Meinersbur

Subscribers: Meinersbur, nemanjai, pollydev, llvm-commits, kbarton

Differential Revision: https://reviews.llvm.org/D39364

llvm-svn: 316872