[OpenMP] Fix handling of clause on wrong directive, by Joel. E. Denny

Summary:
First, this patch fixes an assert failure when, for example, "omp for"
has num_teams.

Second, this patch prevents duplicate diagnostics when, for example,
"omp for" has uniform.

This patch makes the general assumption (even where it doesn't
necessarily fix an existing bug) that it is worthless to perform sema
for a clause that appears on a directive on which OpenMP does not
permit that clause.  However, due to this assumption, this patch
suppresses some diagnostics that were expected in the test suite.  I
assert that those diagnostics were likely just distracting to the
user.

Reviewers: ABataev

Reviewed By: ABataev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41841

llvm-svn: 322107