[DWARF] Extract indexing code into a separate class hierarchy

Summary:
This places the `if(m_using_apple_tables)` branches inside the
SymbolFileDWARF class behind an abstract DWARFIndex class. The class
currently has two implementations:
- AppleIndex, which searches using .apple_names and friends
- ManualIndex, which searches using a manually built index

Most of the methods of the class are very simple, and simply extract the
list of DIEs for the given name from the appropriate sub-table. The main
exception are the two GetFunctions overloads, which take a couple of
extra paramenters, including some callbacks. It was not possible to
split these up the same way as other methods, as here we were doing a
lot of post-processing on the results. The post-processing is similar
for the two cases, but not identical. I hope to factor these further in
separate patches.

Other interesting methods are:
- Preload(): do any preprocessing to make lookups faster (noop for
  AppleIndex, forces a build of the lookup tables for ManualIndex).
- ReportInvalidDIEOffset(): Used to notify the users of an invalid index
  (prints a message for AppleIndex, noop for ManualIndex).
- Dump(): dumps the index state (noop for AppleIndex, prints the lookup
  tables for ManualIndex).

Reviewers: clayborg, JDevlieghere

Subscribers: mgorny, aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D46889

llvm-svn: 332719