Breakpad: Extract parsing code into a separate file

Summary:
This centralizes parsing of breakpad records, which was previously
spread out over ObjectFileBreakpad and SymbolFileBreakpad.

For each record type X there is a separate breakpad::XRecord class, and
an associated parse function. The classes just store the information in
the breakpad records in a more accessible form. It is up to the users to
determine what to do with that data.

This separation also made it possible to write some targeted tests for
the parsing code, which was previously unaccessible, so I write a couple
of those too.

Reviewers: clayborg, lemo, zturner

Reviewed By: clayborg

Subscribers: mgorny, fedor.sergeev, lldb-commits

Differential Revision: https://reviews.llvm.org/D56844

llvm-svn: 351541