[Dwarf] avoid emitting multiple AT_const_value for static memebers.

Testing case is reduced from MultiSource/BenchMarks/Prolangs-C++/deriv1.

rdar://problem/13071590


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174235 91177308-0d34-0410-b5e6-96231b3b80d8