loongarch tests need LLVM version 20 to avoid warnings

Author: RalfJung

tests/ui/target-feature/abi-required-target-feature-flag-disable.rs

@@ -12,6 +12,7 @@
 //@[riscv] needs-llvm-components: riscv
 //@[loongarch] compile-flags: --target=loongarch64-unknown-none -Ctarget-feature=-d
 //@[loongarch] needs-llvm-components: loongarch
+//@[loongarch] min-llvm-version: 20
 // For now this is just a warning.
 //@ build-pass