Update visionOS MachO tests to require LLVM 18

Author: agg23

tests/assembly/targets/targets-macho.rs

@@ -28,9 +28,11 @@
 //@ [arm64_32_apple_watchos] compile-flags: --target arm64_32-apple-watchos
 //@ [arm64_32_apple_watchos] needs-llvm-components: aarch64
 //@ revisions: aarch64_apple_visionos
+//@ [aarch64_apple_visionos] min-llvm-version: 18
 //@ [aarch64_apple_visionos] compile-flags: --target aarch64-apple-visionos
 //@ [aarch64_apple_visionos] needs-llvm-components: aarch64
 //@ revisions: aarch64_apple_visionos_sim
+//@ [aarch64_apple_visionos_sim] min-llvm-version: 18
 //@ [aarch64_apple_visionos_sim] compile-flags: --target aarch64-apple-visionos-sim
 //@ [aarch64_apple_visionos_sim] needs-llvm-components: aarch64
 //@ revisions: arm64e_apple_darwin