[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352358 91177308-0d34-0410-b5e6-96231b3b80d8