[DebugInfo] Avoid name conflict of generated VLA expression variable.

Summary:
This patch also adds the 'DW_AT_artificial' flag to the generated variable.

Addresses the issues mentioned in http://llvm.org/PR30553.

Reviewers: CarlosAlbertoEnciso, probinson, aprantl

Reviewed By: aprantl

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D43189

llvm-svn: 324988