Changes in 226712 needed some fixing as a platform is almost always selected...

Changes in 226712 needed some fixing as a platform is almost always selected and even if platform options are specified when doing a "target create" they would get ignored if a platform was already selected.

The change was made so we could re-use a platform if one was already created instead of creating a new one, but it would fail in the above case. To fix this, if we have a selected platform, we verify that the platform matches the current platform before we try to re-use it. We do this by asking the OptionGroupPlatform if the platform matches. If so, it returns true and we don't create a new platform, else we do.

llvm-svn: 227288