Add ability to get compiler arguments for files generated by build plugin without running build plugin

[ahoppen]

In sourcekit-lsp we want to get the compiler arguments for a source file generated by a build plugin but we definitely don’t want to run the build plugin ourselves (as we don’t have a background build in sourcekit-lsp). So, there would need to be some way by which we can get the compiler arguments for those generated files without actually running the build plugin.

rdar://112122871

[dangdennis]

Out of curiosity, what is the general plan to implementing this fix?