DebugInfo: Suppress the location of instructions in aggregate default arguments.

Matches the existing code for scalar default arguments. Complex default
arguments probably need the same handling too (test/fix to that coming
next).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228588 91177308-0d34-0410-b5e6-96231b3b80d8