Introduce SymbolFileBreakpad and use it to fill symtab

Summary:
This commit adds the glue code necessary to integrate the
SymbolFileBreakpad into the plugin system. Most of the methods are
stubbed out. The only method implemented method is AddSymbols, which
parses the PUBLIC "section" of the breakpad "object file", and fills out
the Module's symtab.

To enable testing this, I've made two additional changes:
- dump Symtab from the SymbolVendor class. The symtab was already being
  dumped as a part of the object file dump, but that happened before
  symbol vendor kicked in, so it did not reflect any symbols added
  there.
- add ability to explicitly specify the external symbol file in
  lldb-test (so that the object file could be linked with the breakpad
  symbol file). To make things simpler, I've changed lldb-test from
  consuming multiple inputs (and dumping their symbols) to having it
  just process a single file per invocation. This was not a problem
  since everyone was using it that way already.

Reviewers: clayborg, zturner, lemo, markmentovai, amccarth

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D56173

llvm-svn: 350924