Check for null pointers before calling the Stmt Profiler

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