[AST] OMPStructuredBlockTest: avoid using multiline string literals in macros

That is what i have been doing elsewhere in these tests, maybe that's it?

Maybe this helps with failing builds:
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-quick/builds/17921
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-global-isel/builds/10248

llvm-svn: 356749