Add a benchmark for creating large uninit allocations

src/tools/miri/bench-cargo-miri/big-allocs/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "big-allocs"
+version = "0.1.0"

src/tools/miri/bench-cargo-miri/big-allocs/Cargo.toml

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+[package]
+name = "big-allocs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/tools/miri/bench-cargo-miri/big-allocs/src/main.rs

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+//! This is a regression test for https://github.com/rust-lang/miri/issues/3637.
+//! `Allocation`s are backed by a `Box<[u8]>`, which we create using `alloc_zeroed`, which should
+//! make very large allocations cheap. But then we also need to not clone those `Allocation`s, or
+//! we end up slow anyway.
+
+fn main() {
+    // We can't use too big of an allocation or this code will encounter an allocation failure in
+    // CI. Since the allocation can't be huge, we need to do a few iterations so that the effect
+    // we're trying to measure is clearly visible above the interpreter's startup time.
+    for _ in 0..10 {
+        drop(Vec::<u8>::with_capacity(512 * 1024 * 1024));
+    }
+}