[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@372318
91177308-0d34-0410-b5e6-
96231b3b80d8