These tests were all using WpfFact, but not actually doing anything
that required either WPF or an STA thread. We plan to enable
parallelization of Facts (but not WpfFacts), and this ensures we can
parallelize as many tests as possible.

Avoid repeatedly calling ISyntaxFactsService.IsInInactiveRegion, which calls SyntaxTree.GetToken. GetToken needs to traverse directive trivia to find the token it is
attached to. This results in n ^ 2 traversals of the tree if there are repeated directive before the line to indent. Additionally, when the indenter determines
that it's examining a line inside a disabled region, instead of examining the previous line, it examines the first line preceeding the current line that isn't
inside a disabled region.

Fixes #5394.

* Convert the ICompletionProvider interface to a CompletionListProvider abstract class.
* Rename CompletionItemGroup to CompletionList
* Introduce a CompletionListContext type to be used by providers to specify the contents and
  details of CompletionLists with a RegisterCompletionListAsync() method.
* Change GetGroupsAsync() method to GetCompletionListAsync(), which returns Task<CompletionList> rather
  than Task<IEnumerable<CompletionList>>.
* Move code that merges, de-dupes and sorts completion lists from the controller into the
  completion service. This affected a few VB spell check unit tests which now change their ordering
  slightly.

Previously, we would fetch a SemanticModel and build containing Symbols for the current block.  This could be very expensive in a large solution.  However, for the purposes of this scenario, it is adequate to just inspect Syntax.

This change also unifies the implementation of IVsLanguageBlock.GetCurrentBlock with IVsLanguageDebugInfo.GetNameOfLocation.

Fixes #714.