fix Issue 15047 - "used before set" error with -O

[Cauterite]

Currently there is a case where the optimiser is able to detect an error that is not otherwise detected. This results in a compilation error only when the -O flag is used.
If code is buildable with -O it should be buildable without -O. Therefore, the optimiser now emits a warning instead.

[dlang-bot]

Fix	Bugzilla	Description
✗	15047	"used before set" error with void initialization (only with -O)

[dlang-bot]

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[Cauterite]

Currently the -w flag is broken with ddoc. If ddoc prints warnings, compilation continues regardless of the -w flag.

With that fixed, certain ddoc tests are failing because they trigger warnings and specify the -w flag.

[Cauterite]

Looks like fixing the -w flag has a much wider effect than I had expected. Therefore I'm going to split this pull request into two.

TLDR: -w is a very broken feature right now.