Proposal: Compilation Pipeline

[alrz]

Generators (#5561) allow to add code to the compilation unit and replace/origin (#5292) can replace members at the declaration-site. But still we can't intercept statements or method calls that are potentially outside of the assembly boundaries (e.g. linq calls could be inlined). This is not just about the code itself, one could be able to run an analysis on the code and generate useful information in the resultant xml docs (#15573). I don't know if currently there exists a mechanism to do this. But I'd greatly appreciate it if we could intercept the compilation pipeline and access to all the information that the compiler produces. This could be an alternative to il weaving, except for that it's done before the actual codegen.

[alrz]

Closing per #5561 (comment).