[AMDGPU] Add interpolation builtins

Summary:
Added builtins for the interpolation intrinsics, and related LIT
test.

Reviewers: arsenm, tpr, dstuttard, #amdgpu

Reviewed By: arsenm, #amdgpu

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, cfe-commits

Differential Revision: https://reviews.llvm.org/D46871

llvm-svn: 352358