Rollup merge of rust-lang#89591 - infinity0:master, r=Amanieu

fix: alloc-optimisation is only for rust llvm

As discussed at the bottom of rust-lang#83485.

On a separate note I'll take this chance ask, is it worth pulling in that patch (to recognise `__rust_dealloc`) into Debian's system LLVM? The main factors for us to consider would be (1) is the optimisation significant and (2) is there not any significant negative impact to non-rust packages that use LLVM.

Author: Manishearth

src/test/codegen/alloc-optimisation.rs

@@ -1,4 +1,5 @@
 //
+// no-system-llvm
 // min-llvm-version: 10.0.1
 // compile-flags: -O
 #![crate_type="lib"]