This is step 2 towards fixing #77548.
In the codegen and codegen-units test suites, the `//` comment markers
were kept in order not to affect any source locations. This is because
these tests cannot be automatically `--bless`ed.
This will make @path work with miri and other non-standard entrypoints.
Also since this simplifies librustc_driver::args, move it into a simple source file. Also
remove the tests since they're doing nothing more than checking `str::lines` has the right
behaviour.
This makes `rustc` support `@path` arguments on the command line. The `path` is opened and the file is interpreted
as new command line options which are logically inserted at that point in the command-line. The options in the file
are one per line. The file is UTF-8 encoded, and may have either Unix or Windows line endings.
It does not support recursive use of `@path`.
This is useful for very large command lines, or when command-lines are being generated into files by other tooling.
