Add the ability for DWARFDie objects to get the parent DWARFDie.

Add the ability for DWARFDie objects to get the parent DWARFDie.

In order for the llvm DWARF parser to be used in LLDB we will need to be able to get the parent of a DIE. This patch adds that functionality by changing the DWARFDebugInfoEntry class to store a depth field instead of a sibling index. Using a depth field allows us to easily calculate the sibling and the parent without increasing the size of DWARFDebugInfoEntry.

I tested llvm-dsymutil on a debug version of clang where this fully parses DWARF in over 1200 .o files to verify there was no serious regression in performance.

Added a full suite of unit tests to test this functionality.

Differential Revision: https://reviews.llvm.org/D27995

llvm-svn: 290274