Fix for PR30639: CGDebugInfo Null dereference with OpenMP array

access, by Erich Keane

OpenMP creates a variable array type with a a null size-expr. The Debug
generation failed to due to this. This patch corrects the openmp
implementation, updates the tests, and adds a new one for this
condition.

Differential Revision: https://reviews.llvm.org/D25373

llvm-svn: 284110