Allow #[rustc_builtin_macro = "name"].

This makes it possible to have both std::panic and core::panic as a
builtin macro, by using different builtin macro names for each.

Also removes SyntaxExtension::is_derive_copy, as the macro name (e.g.
sym::Copy) is now tracked and provides that information directly.

compiler/rustc_builtin_macros/src/lib.rs

@@ -48,7 +48,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand, edition: Editi
     let mut register = |name, kind| {
         resolver.register_builtin_macro(
             Ident::with_dummy_span(name),
-            SyntaxExtension { is_builtin: true, ..SyntaxExtension::default(kind, edition) },
+            SyntaxExtension { builtin_name: Some(name), ..SyntaxExtension::default(kind, edition) },
         )
     };
     macro register_bang($($name:ident: $f:expr,)*) {