Remove TmpLayout in layout_of_enum

[zachs18]

09a3846 from #103693 made LayoutData be owned instead of interned in Variants::Multiple::variants¹, so there's no need for TmpLayout in layout_of_enum anymore, and we can just store the variants' layouts directly in the prospective LayoutDatas' variants fields.

This should have no effect on semantics or layout.

(written as part of #145337 but not related to the layout optimizations in that PR)

Footnotes

1. see line 1154 of compiler/rustc_target/src/abi/mod.rs in the linked commit; Variants::Multiple::variants effectively changed from IndexVec<.., Layout<'tcx>> to IndexVec<.., LayoutData> where the LayoutDatas are not interned as Layouts (LayoutData was at the time called LayoutS) ↩

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