[DebugInfo] Emit DWARF reference for DIVariable 'count' in DISubrange

Summary:
This patch implements the codegen of DWARF debug info for non-constant
'count' fields for DISubrange.

This is patch [2/3] in a series to extend LLVM's DISubrange Metadata
node to support debugging of C99 variable length arrays and vectors with
runtime length like the Scalable Vector Extension for AArch64. It is
also a first step towards representing more complex cases like arrays
in Fortran.

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

Reviewed By: aprantl

Subscribers: fhahn, aemerson, rengolin, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D41696

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323323 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed