[DWARF] Reimplement/simplify DWARFUnit::GetID

Summary:
The implementation of GetID used a relatively complicated algorithm,
which returned some kind of an offset of the unit in some file
(depending on the debug info flavour). The only thing this ID was used
for was to enable subseqent retrieval of the unit from the SymbolFile.

This can be made simpler if we just make the "ID" of the unit an index
into the list of the units belonging to the symbol file. We already
support indexed access to the units, so each unit already has a well
"index" -- this just makes it accessible from within the unit.

To make the distincion between "id" and "offset" clearer (and help catch
any misuses), I also rename DWARFDebugInfo::GetCompileUnit (which
accesses by offset) into DWARFDebugInfo::GetCompileUnitAtOffset.

On its own, this only brings a minor simplification, but it enables
further simplifications in the DIERef class (coming in a follow-up
patch).

Reviewers: JDevlieghere, clayborg, aprantl

Subscribers: arphaman, jdoerfert, lldb-commits, tberghammer, jankratochvil

Differential Revision: https://reviews.llvm.org/D61481

llvm-svn: 360014