Rollup merge of #30823 - pnkfelix:put-back-alloca-zeroing-for-issue-30530, r=dotdash

Put back alloca zeroing for issues #29092, #30018, #30530; inject zeroing for #30822.

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Background context: `fn alloca_zeroed` was removed in PR #22969, so we haven't been "zero'ing" (\*) the alloca's since at least that point, but the logic behind that PR seems sound, so its not entirely obvious how *long* the underlying bug has actually been present.  In other words, I have not yet done a survey to see when the new `alloc_ty` and `lvalue_scratch_datum` calls were introduced that should have had "zero'ing" the alloca's.

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I first fixed #30018, then decided to do a survey of `alloc_ty` calls to see if they needed similar treatment, which quickly led to a rediscovery of #30530.

While making the regression test for the latter, I discovered #30822, which is a slightly different bug (in terms of where the "zero'ing" needs to go), but still relevant.

I haven't finished the aforementioned survey of `fn alloc_ty` calls, but I decided I wanted to get this up for review in its current state (namely to see if my attempt to force developers to include a justification for passing `Uninit` can possibly fly, or if I should abandon that path of action).

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(*): I am putting quotation marks around "zero'ing" because we no longer use zero as our "dropped" marker value.

Fix #29092
Fix #30018
Fix #30530
Fix #30822

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