[WIP][DebugInfo] Lazily parse debug_loclist offsets

[WIP][DebugInfo] Lazily parse debug_loclist offsets

Parsing DWARFv5 debug_loclist offsets when a CU is parsed is weighing
down memory usage of symbolizers that don't need to parse this data at
all. There's not much benefit to caching these anyway - since they are
O(1) lookup and reading once you know where the offset list starts (and
can do bounds checking with the offset list size too).

In general, I think it might be time to start paying down some of the
technical debt of loc/loclist/range/rnglist parsing to try to unify it a
bit more.

eg:

* Currently DWARFUnit has: RangeSection, RangeSectionBase, LocSection,
  LocSectionBase, LocTable, RngListTable, LoclistTableHeader (be nice if
  these were all wrapped up in two variables - one for loclists, one for
  rnglists)

* rnglists and loclists are handled differently (see:
  LoclistTableHeader, but no RnglistTableHeader)

* maybe all these types could be less stateful - lazily parse what they
  need to, even reparsing rather than caching because it doesn't seem
  too expensive, for instance. (though admittedly so long as it's
  constantcost/overead per compilatiton that's probably adequate)

* Maybe implementing and using a DWARFDataExtractor that can be
  sub-ranged (so we could slice it up to just the single contribution) -
  though maybe that's not so useful because loc/ranges need to refer to
  it by absolute, not contribution-relative mechanisms

Differential Revision: https://reviews.llvm.org/D86110