[OPENMP50]Do not allow multiple same context traits in the same context
selector.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374093 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed