Disable windows-only minidump plugin

Summary:
This commit disables the windows-only minidump plugin and enables the new
cross-platform plugin for windows minidump files. Test decorators are adjusted to
reflect that: windows minidump tests can now run  on all platforms. The exception
is the tests that create minidump files, as that functionality is not available
yet.  I've checked that this works on windows and linux.

Reviewers: amccarth, zturner

Subscribers: dvlahovski, lldb-commits

Differential Revision: https://reviews.llvm.org/D26393

llvm-svn: 286352