[AccelTable] Provide DWARF5AccelTableStaticData for dsymutil.

For dsymutil we want to store offsets in the accelerator table entries
rather than DIE pointers. In addition, we need a way to communicate
which CU a DIE belongs to. This patch provides support for both of these
issues.

Differential revision: https://reviews.llvm.org/D49102

llvm-svn: 337158