Reduce allocations in TextDocumentStates.SelectAs* methods

[ToddGrun]

These methods were using an unsized ArrayBuilder to create an ImmutableArray.

Instead, as the size is pre-known in all these methods, we can just use ImmutableArray.CreateBuilder directly.

These two methods were showing up as a combined 0.4% of allocations in the codeanalysis process when inspecting a profile taken while typing a line of text.

Name                                                        	                                                                         Inc %	    Inc
Microsoft.CodeAnalysis.Workspaces!Microsoft.CodeAnalysis.TextDocumentStates`1[System.__Canon].SelectAsArray(class System.Func`3,!!1) 	  0.3	   39,169,344
Microsoft.CodeAnalysis.Workspaces!Microsoft.CodeAnalysis.TextDocumentStates`1[System.__Canon].SelectAsArray(class System.Func`2) 	  0.1	   9,204,744

[Youssef1313]

Previously this was reusing instances from a pool. Now, this will allocate each time.

[ToddGrun]

A couple thoughts:

1. Previously, the code called builder.ToImmutable every time, which allocates.
2. Previously the code would incrementally resize by calling Add. This could cause multiple re-allocations before reaching it's final size. (and have it's final size be up to twice too large)
3. Previously the code would end up not adding the builder back to the pool if it exceeded the size threshold (fairly common in the scenario I'm testing)
4. Previously, the code would need to do the overhead of searching the ObjectPool for an object to use.

[Youssef1313]

Makes sense!