Adding a new Minidump post-mortem debugging plugin

Summary:
This plugin resembles the already existing Windows-only Minidump plugin.
The WinMinidumpPlugin uses the Windows API for parsing Minidumps
while this plugin is cross-platform because it includes a Minidump
parser (which is already commited)

It is able to produce a backtrace, to read the general puprose regiters,
inspect local variables, show image list, do memory reads, etc.

For now the only arch that this supports is x86 64 bit
This is because I have only written a register context for that arch.
Others will come in next CLs.

I copied the WinMinidump tests and adapted them a little bit for them to
work with the new plugin (and they pass)
I will add more tests, aiming for better code coverage.

There is still functionality to be added, see TODOs in code.

Reviewers: labath, zturner

Subscribers: beanz, mgorny, amccarth, lldb-commits, modocache

Differential Revision: https://reviews.llvm.org/D25196

llvm-svn: 283259