[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

llvm-svn: 268870