[DebugInfo] Enable debug information for C99 VLA types

[DebugInfo] Enable debug information for C99 VLA types

Summary:
This patch enables debugging of C99 VLA types by generating more precise
LLVM Debug metadata, using the extended DISubrange 'count' field that
takes a DIVariable.

This should implement:
  Bug 30553: Debug info generated for arrays is not what GDB expects (not as good as GCC's)
https://bugs.llvm.org/show_bug.cgi?id=30553

Reviewers: echristo, aprantl, dexonsmith, clayborg, pcc, kristof.beyls, dblaikie

Reviewed By: aprantl

Subscribers: jholewinski, schweitz, davide, fhahn, JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D41698

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@323952 91177308-0d34-0410-b5e6-96231b3b80d8