Incremental compilation and codegen that produces no object files

[pacak]

It is possible to ask rustc to produce intermediate files (assembly, llvm-ir) without asking to produce object files. This works fine as long as incremental compilation is disabled. Once it is enabled - it fails in all sorts of ways - multiple asserts and with those removed - it fails to restore intermediate product from the cache since they are not cached (#123221 deals with that).

rm -rf *
touch lib.rs
rustc --emit asm --crate-type lib lib.rs -C incremental=foo
rustc --emit asm --crate-type lib lib.rs -C incremental=foo 

Fails with this (and more)

thread 'cpy tzqeezh2sedvpj0' panicked at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/compiler/rustc_codegen_ssa/src/back/write.rs:890:5:
assertion failed: module_config.emit_obj != EmitObj::None

Options are:

1. remove the asserts or allow them to pass if user never asked for object code in the first place
2. change the logic such that requesting for byproducts implies request for object files