Allow pre-processor directives in XML comments

[TaffarelJr]

Currently the lexical specification does not allow elements like pre-processor directives to be interspersed with XML documentation. This means we get compiler errors when trying to do something like this:

Originally filed in dotnet/roslyn #70057

I believe it is a valid scenario to want to have specific XML documenation for different platforms when multi-targeting, without having to duplicate entire blocks of XML comments.

What is the feasibility of such a feature being supported?

[CyrusNajmabadi]

Ok a technical level it would be extremely challenging. These are different trivia structures, so the content would be be split. The XML system would then have to be rewritten to not have everything be contiguous.

[jnm2]

Existing discussion: #295