FR: make disabling `format_generated_files` work with stdin input

Currently, rustfmt formats generated files if they are passed via stdin, even if format_generated_files=false. I think it's nice to have an issue tracking this problem specifically.

See https://github.com/rust-lang/rustfmt/issues/5080#issuecomment-1500232131 for an overview with a few more details, and https://github.com/rust-lang/rustfmt/issues/5172 for the reason for the current behavior.

IMO, if format_generated_files=false, rustfmt should act like cat on generated files passed via stdin. I'm unsure whether there are any issues with such a plan. This is already what happens when disable_all_formatting=true.

I'll link to https://github.com/rust-lang/rustfmt/issues/5137 which I think is spiritually related; if that were fixed, it would be a workaround for this issue as well (and vice-versa, as far as generated files are concerned, which is currently my main use-case for #5137).

Start by reviewing rustfmt's stdin handling and the interaction between format_generated_files=false and generated-file detection. Read the linked discussions in issues 5080, 5172, and 5137 for the current behavior and constraints; done means generated files received through stdin are passed through unchanged when formatting is disabled.