[OpenMP] DSAChecker bug fix for combined directives.

The DSAChecker code in SemaOpenMP looks at the captured statement
associated with an OpenMP directive.  A combined directive such as
'target parallel' has nested capture statements, which have to be
fully traversed before executing the DSAChecker.  This is a patch
to perform the traversal for such combined directives.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29026

llvm-svn: 292794