Rollup merge of #79077 - RalfJung:llvm-magic, r=Mark-Simulacrum

document that __rust_alloc is also magic to our LLVM fork

Based on [comments](https://github.com/rust-lang/rust/pull/79045#discussion_r523442198) by ````@tmiasko```` and ````@bjorn3.````

library/alloc/src/alloc.rs

@@ -23,6 +23,8 @@
     // (the code expanding that attribute macro generates those functions), or to call
     // the default implementations in libstd (`__rdl_alloc` etc. in `library/std/src/alloc.rs`)
     // otherwise.
+    // The rustc fork of LLVM also special-cases these function names to be able to optimize them
+    // like `malloc`, `realloc`, and `free`, respectively.
     #[rustc_allocator]
     #[rustc_allocator_nounwind]
     fn __rust_alloc(size: usize, align: usize) -> *mut u8;