Make `astyle` under `:ALEFix` operate on buffer

[ankitpati]

Previously, :ALEFix made astyle operate on the disk file, and populated the buffer with the results, potentially losing work.

This PR regresses #3257, but that appears to be a bug with astyle itself, not with ale, so it should be fixed there anyways. I feel avoiding data-loss should be a higher goal than working around astyle bugs.

Thanks for your awesome work!

[ankitpati]

@jhlink Steps to reproduce:

1. Ensure astyle is the only ALE fixer defined for C.
2. Open a .c file with valid C code.
3. Add a line of valid C code anywhere in the file.
4. Run :ALEFix
5. Observe that the added line has vanished.

[hsanson]

Took some time to re-read the related discussions and test this PR and unfortunately applying this PR reverts the initial issue. If we do not use the --stdin flag then astyle fails to find and use any .astylerc or _astylerc configuration files within the project. This would be a breaking change so I cannot approve this PR unless alternative solution for the .astylerc/_astylerc file issue is also included.

[jhlink]

Thanks for helping with the analysis @hsanson !

[stale[bot]]

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