Auto merge of #88832 - pcwalton:debug-unit-variant-fast-path, r=oli-obk

Introduce a fast path that avoids the `debug_tuple` abstraction when deriving Debug for unit-like enum variants.

The intent here is to allow LLVM to remove the switch entirely in favor of an
indexed load from a table of constant strings, which is likely what the
programmer would write in C. Unfortunately, LLVM currently doesn't perform this
optimization due to a bug, but there is [a
patch](https://reviews.llvm.org/D109565) that fixes this issue. I've verified
that, with that patch applied on top of this commit, Debug for unit-like tuple
variants becomes a load, reducing the O(n) code bloat to O(1).

Note that inlining `DebugTuple::finish()` wasn't enough to allow LLVM to
optimize the code properly; I had to avoid the abstraction entirely. Not using
the abstraction is likely better for compile time anyway.

Part of #88793.

r? `@oli-obk`

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