NFC: Refactor ProcessWinMiniDump to use a more traditional pimpl idiom.

This is a mechanical refactor.  There should be no functional changes in this commit.

Instead of encapsulating just the Windows-specific data, ProcessWinMiniDump now uses a private implementation class.  This reduces indirections (in the source).  It makes it easier to add private helper methods without touching the header and allows them to have platform-specific types as parameters.  The only trick was that the pimpl class needed a back pointer in order to call a couple methods.

llvm-svn: 262256