Refactor out the abbreviation handling into a separate class that

Refactor out the abbreviation handling into a separate class that
controls each of the abbreviation sets (only a single one at the
moment) and computes offsets separately as well for each set
of DIEs.

No real function change, ordering of abbreviations for the skeleton
CU changed but only because we're computing in a separate order. Fix
the testcase not to care.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169793 91177308-0d34-0410-b5e6-96231b3b80d8