[CUDA][HIP] Fix typo in `BestViableFunction`

Summary:
- Should consider viable ones only when checking SameSide candidates.
- Replace erasing with clearing viable flag to reduce data
  moving/copying.
- Add one and revise another one as the diagnostic message are more
  relevant compared to previous one.

Reviewers: tra

Subscribers: cfe-commits, yaxunl

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67730

llvm-svn: 372318