[OpenMP] Check for associated statements with hasAssociatedStmt() when scanning for device code.
Summary: `getAssociatedStmt()` contains an assertion that assumes the statement always exists. In device code scanning, we need to look into the associated statement therefore we check its existence. This patch replaces `getAssociatedStmt` by `hasAssociatedStmt` so that we do not trigger the assertion for directives that happen not to have an associated statement (e.g target enter/exit data).
Reviewers: hfinkel, carlo.bertolli, arpith-jacob, kkwli0, ABataev
Subscribers: cfe-commits, caomhin
Differential Revision: http://reviews.llvm.org/D19812
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268870
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