Fixes #3092 - Implement loading @file c arguments

[sudobash1]

This is a fix for the bug #3092 I filed on expanding @file c arguments.

Just as background, a @file argument works like this: If gcc or clang encounters a flag which starts with @ it will treat the remainder of that flag as a file path, and it will use the contents of that file as flags. (It is somewhat like an "include file" for command line arguments).

Initially I implemented this as simply passing the @file arguments directly to the compiler, but then I realized that this could cause problems since the arguments contained with the file would not get parsed and filtered. If you are curious this implementation is here.

This PR instead has vim read the contents of the @file and will add those to the argument list to be filtered.

I have added a Vader test for this.

Thanks for you work!

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[w0rp]

I say we just give this a shot and see if anyone notices any issues. I can't think of a better way to handle this.

[w0rp]

Cheers! 🍻

[w0rp]

If we end up with someone having issues with readfile() calls being too slow, we have the option later of storing the result of readfile() for @file arguments in a Dictionary, so we don't read the file from disk all the time.

[sudobash1]

I think this is my first open-source PR to get merged. Thanks for your work on this project @w0rp.