[DWARF] Add (empty) DebugNamesDWARFIndex class and a setting to control its use

Summary:
This patch adds the skeleton for implementing the DWARF v5 name index
class. All of the methods are stubbed out and will be implemented in
subsequent patches. The interesting part of the patch is the addition of
a "ignore-file-indexes" setting to the dwarf plugin which enables a
user to force using manual indexing path in lldb (for example as a
debugging aid). I have also added a test that verifies that file indexes
are used by default.

Reviewers: JDevlieghere, clayborg, jingham

Subscribers: mgorny, mehdi_amini, aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D47629

llvm-svn: 334088