DWARFAcceleratorTable: Add an iterator-based api for accessing names in the index

Summary:
Back when we were introducing the DWARF v5 name index, there was a
short discussion whether we shouldn't have a nicer api for iterating
over the index. At that time, I did not find it necessary since the
iteration over names was done only from within the index itself (and I
figured the internal implementation can deal with a slightly rough
interface).

However, now I ran into a use for this kind of API in LLDB (for finding
all names matching a regular expression), so it looked like a nice
opportunity to introduce one. To make the API more useful, I've made the
NameTableEntry class a bit smarter: it now stores the string section
reference (so it can return its name) and its position in the name index
(mainly useful for dumping/logging).

I also convert the internal users to use the new API, which also gives
test coverage for the added code.

Reviewers: JDevlieghere, aprantl, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47590

llvm-svn: 333738