Check for null pointers before calling the Stmt Profiler

Some calls from OMPClauseProfiler were calling the Stmt Profiler with null
pointers, but the profiler can only handle non-null pointers.  Add an assert
to the VisitStmt for valid pointers, and check all calls from OMPClauseProfiler
to be non-null pointers.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272368 91177308-0d34-0410-b5e6-96231b3b80d8