Enable -preview=fieldwise by default

[pbackus]

Introduced in PR #9331, this was changed from a normal bug fix to a -preview switch because enabling it triggered a suprious "statement is not reachable" warning in Phobos. That warning has since been removed (in PR #15568).

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[ibuclaw]

@pbackus - this doesn't do what you think it does. The changelog entry should be removed to avoid confusion, or really make -preview=fieldwise the default.

[pbackus]

@ibuclaw In the future, when you notice a mistake in a PR, the most helpful thing to do would be to point out the mistake directly.

[ibuclaw]

@pbackus noted - I did post a little in haste.