[OpenMP] Fix formatting of OpenMP error message, by Wang Tianqing.

Summary: `getListOfPossibleValues()` formatted incorrectly when there is only one value, emitting something like `expected 'conditional' or  in OpenMP clause 'lastprivate'`.

Reviewers: jdoerfert, ABataev

Reviewed By: jdoerfert

Subscribers: guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71884