Fix sign extension handling in DumpEnumValue
Fix sign extension handling in DumpEnumValue

When an enumerator has an unsigned type and its high bit set, the
code introduced in r374067 would fail to match it due to a sign
extension snafu. This commit fixes this aspec of the code and should
fix the bots.

I think it's not a complete fix though, I'll add more test coverage
and additional tweaks in a follow-up commit.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@374095 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 72a88b91c74d7ba9a3ce5a606ccc70c977616967)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb