[dsymutil] Skip DW_AT_sibling attributes.

Following DW_AT_sibling attributes completely defeats the pruning pass.
Although clang doesn't generate the DW_AT_sibling attribute we should
still handle it correctly.

Differential revision: https://reviews.llvm.org/D43439

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326231 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed