[DWARFv5] Put the DWO ID in its place.

In DWARF v5, the DWO ID is in the (split/skeleton) CU header, not an
attribute on the CU DIE.

This changes the size of those headers, so use the parsed size whenever
we have one, for simplicitly.

Differential Revision: https://reviews.llvm.org/D47158

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333004 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed