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.