ManageCache method, avoid Task.Wait

[lsoft]

I believe it's the safer alternative to use Task.Wait:

        public CacheState ManageCache(ref ProviderCache cache, NormalizedSnapshotSpanCollection spans, ITextBuffer buffer)
        {
            if (cache != null && cache.Snapshot == spans[0].Snapshot)
            {
                return CacheState.Resolved;
            }

            ProviderCache cache1 = null;

            var result = ThreadHelper.JoinableTaskFactory.Run<CacheState>(async () =>
            {
                await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();

                try
                {
                    cache1 = await ProviderCache.ResolveAsync(buffer, spans[0].Snapshot);
                    return cache1 == null ? CacheState.NotResolved : CacheState.Resolved;
                }
                catch (Exception ex)
                {
                    Log.LogError(ex.ToString());
                    return CacheState.NotResolved;
                }
            });

            cache = cache1;
            return result;
        }