ObjectFileBreakpad: Implement sections

Summary:
This patch allows ObjectFileBreakpad to parse the contents of Breakpad
files into sections. This sounds slightly odd at first, but in essence
its not too different from how other object files handle things. For
example in elf files, the symtab section consists of a number of
"records", where each record represents a single symbol. The same is
true for breakpad's PUBLIC section, except in this case, the records will be
textual instead of binary.

To keep sections contiguous, I create a new section every time record
type changes. Normally, the breakpad processor will group all records of
the same type in one block, but the format allows them to be intermixed,
so in general, the "object file" may contain multiple sections with the
same record type.

Reviewers: clayborg, zturner, lemo, markmentovai, amccarth

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D55434

llvm-svn: 350511