Auto merge of #94130 - erikdesjardins:partially, r=nikic

Use undef for (some) partially-uninit constants

There needs to be some limit to avoid perf regressions on large arrays
with undef in each element (see comment in the code).

Fixes: #84565
Original PR: #83698

Depends on LLVM 14: #93577

compiler/rustc_middle/src/mir/interpret/allocation.rs

@@ -957,6 +957,7 @@ impl InitMask {
 }
 
 /// Yields [`InitChunk`]s. See [`InitMask::range_as_init_chunks`].
+#[derive(Clone)]
 pub struct InitChunkIter<'a> {
     init_mask: &'a InitMask,
     /// Whether the next chunk we will return is initialized.