[AMDGPU] Allow using integral non-type template parameters

Summary:
- Allow using integral non-type template parameters in the following
  attributes

  __attribute__((amdgpu_flat_work_group_size(<min>, <max>)))
  __attribute__((amdgpu_waves_per_eu(<min>[, <max>])))

Reviewers: kzhuravl, yaxunl

Subscribers: jvesely, wdng, nhaehnle, dstuttard, tpr, t-tye, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58623

llvm-svn: 354909