Always emit DW_AT_declaration attribute when the variable isn't a definition.

Summary:
This doesn't show up today as we don't emit decalration only variables. This
will be tested when the followup patches implementing import of forward
declared entities lands in clang.

Reviewers: echristo, dblaikie, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5382