[OPENMP50]Prohibit multiple context selector sets in context selectors.

According to OpenMP 5.0, 2.3.2 Context Selectors, Restrictions, each
trait-set-selector-name can only be specified once. Added check to
implement this restriction.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374072 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed