[Object] Add basic minidump support

Summary:
This patch adds basic support for reading minidump files. It contains
the definitions of various important minidump data structures (header,
stream directory), and of one minidump stream (SystemInfo). The ability
to read other streams will be added in follow-up patches. However, all
streams can be read even now as raw data, which means lldb's minidump
support (where this code is taken from) can be immediately rebased on
top of this patch as soon as it lands.

As we don't have any support for generating minidump files (yet), this
tests the code via unit tests with some small handcrafted binaries in
the form of c char arrays.

Reviewers: Bigcheese, jhenderson, zturner

Subscribers: srhines, dschuff, mgorny, fedor.sergeev, lemo, clayborg, JDevlieghere, aprantl, lldb-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59291

llvm-svn: 356652