Add a mechanism for forcing codegen for an import/module.

[just-harry]

This PR adds a mechanism which lets the frontend indicate that codegen should always take place for an import/module, even if that module wasn't supplied on the command-line when the compiler was invoked, and regardless of whether the -i switch was supplied or not.

My main motivation for this is for #16372; so that codegen can be forced for the __builtins_msvc module, so that #include <importc_msvc_builtins.h> just works regardless of how the compiler was invoked. (Additional context: #16372 (comment); #16372 (comment); #16372 (comment))

[rikkimax]

This could very easily lead to duplicate symbols.

All the symbols would need to have the appropriate linker flag(s) set to allow for duplicates.

As-is this will result in our builds being more fragile than necessary. Some thinking about how to handle duplicates needs to take place before merging a solution to this.

@kinke and @ibuclaw will also need to sign off on this.

[just-harry]

Closing this as I've realised a way to obviate the need for forcing module-wide codegen for #16372