DebugInfo: Use the scope of the function declaration, if any, to name a function in DWARF pubnames

This ensures that member functions, for example, are entered into
pubnames with their fully qualified name, rather than inside the global
namespace.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210379 91177308-0d34-0410-b5e6-96231b3b80d8