Recommend preprocessing symbols that have already been typed out in the document

Right now, typing out a preprocessing directive does not recommend any symbols aside from the standard .NET SDK ones. So if I have already typed #if SYMBOL in the document, I should be getting it recommended in expressions inside #define, #undef, #if, #elif.

The restriction for symbols within the document is only to avoid performance pitfalls, if there is no index capturing all the preprocessing symbol identifiers used in the compilation. Otherwise, it would be nice to extend this globally across the entire assembly or compilation.