use declarative macro for #[derive(TryFromU32)]

compiler/rustc_codegen_llvm/src/macros.rs

@@ -0,0 +1,22 @@
+macro_rules! TryFromU32 {
+    derive() (
+        $(#[$meta:meta])*
+        $vis:vis enum $Type:ident {
+            $(
+                $(#[$varmeta:meta])*
+                $Variant:ident $(= $discr:expr)?
+            ),* $(,)?
+        }
+    ) => {
+        impl ::core::convert::TryFrom<u32> for $Type {
+            type Error = u32;
+            #[allow(deprecated)] // Don't warn about deprecated variants.
+            fn try_from(value: u32) -> ::core::result::Result<$Type, Self::Error> {
+                $( if value == const { $Type::$Variant as u32 } { return Ok($Type::$Variant) } )*
+                Err(value)
+            }
+        }
+    }
+}
+
+pub(crate) use TryFromU32;