Implement equal_range for the DWARF v5 accelerator table

Summary:
This patch implements the name lookup functionality of the .debug_names
accelerator table and hooks it up to "llvm-dwarfdump -find". To make the
interface of the two kinds of accelerator tables more consistent, I've
created an abstract "DWARFAcceleratorTable::Entry" class, which provides
a consistent interface to access the common functionality of the table
entries (such as getting the die offset, die tag, etc.). I've also
modified the apple table to vend entries conforming to this interface.

Reviewers: JDevlieghere, aprantl, probinson, dblaikie

Subscribers: vleschuk, clayborg, echristo, llvm-commits

Differential Revision: https://reviews.llvm.org/D43067

llvm-svn: 326003