Minidump/Windows: Fix module lookup

Summary:
When opening a minidump, we were failing to find an executable because
we were searching for i386-unknown-windows, whereas we recognize the
pe/coff files as i386-pc-windows. This fixes the triple computation code
in the minidump parser to match pe/coff, and adds an appropriate test.

NB: I'm not sure setting the vendor to "pc" is really correct for
arm(64) windows, but right now that seems to match what we do in the
pe/coff case (ArchSpec.cpp:935).

Reviewers: clayborg, amccarth

Subscribers: javed.absar, kristof.beyls, rnk, markmentovai, lldb-commits

Differential Revision: https://reviews.llvm.org/D65955

llvm-svn: 368416