Minidump parsing

Summary:
This is a Minidump parsing code.
There are still some more structures/data streams that need to be added.
The aim ot this is to be used in the implementation of
a minidump debugging plugin that works on all platforms/architectures.
Currently we have a windows-only plugin that uses the WinAPI to parse
the dump files.
Also added unittests for the current functionality.

Reviewers: labath, amccarth

Subscribers: tberghammer, danalbert, srhines, lldb-commits, dschuff

Differential Revision: https://reviews.llvm.org/D23545

llvm-svn: 280356