DWARF: Move indexing code from DWARFUnit to ManualDWARFIndex

Summary:
I think this makes sense for several reasons:
- better separation of concerns: DWARFUnit's job should be to provide a
  nice interface to its users to access the unit contents.
  ManualDWARFIndex can then use this interface to build an index and
  provide it to its users.
- closer alignment with llvm parsers: there is no indexing equivalent in
  llvm, and there probably never will be, as the index is very centered
  around how lldb wants to access debug info. If we ever switch to
  llvm's parser, this will allow us swap out DWARFUnit implementations
  and keep indexing as-is.
- closer proximity of the indexing code to AppleDWARFIndex will make it
  easier to keep the two in sync (e.g. right now the two use very
  different algorithms to determine whether a DW_TAG_subroutine
  represents a "method"). This is my primary motivation for making this
  change now, but I am leaving this work to a separate patch.

The only interface change to DWARFUnit I needed to make was to add an
efficient way to iterate over the list of all DIEs. Adding this also
aligns us closer to the llvm parser.

Reviewers: JDevlieghere, clayborg, aprantl

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D47253

llvm-svn: 333178