Minidump plugin: Adding x86_32 register context converter

Summary:
This, like the x86_64 case, reads the register values from the minidump
file, and emits a binary buffer that is ordered using the offsets from
the RegisterInfoInterface argument. That way we can reuse an existing
register context.
Added unit tests.

Reviewers: labath, zturner

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

Differential Revision: https://reviews.llvm.org/D25832

llvm-svn: 285584