Implement #[define_opaque] attribute for functions.

compiler/rustc_error_codes/src/error_codes/E0792.md

@@ -7,6 +7,7 @@ This means
 
 type Foo<T> = impl std::fmt::Debug;
 
+#[define_opaque(Foo)]
 fn foo() -> Foo<u32> {
     5u32
 }
@@ -19,6 +20,7 @@ is not accepted. If it were accepted, one could create unsound situations like
 
 type Foo<T> = impl Default;
 
+#[define_opaque(Foo)]
 fn foo() -> Foo<u32> {
     5u32
 }
@@ -36,6 +38,7 @@ Instead you need to make the function generic:
 
 type Foo<T> = impl std::fmt::Debug;
 
+#[define_opaque(Foo)]
 fn foo<U>() -> Foo<U> {
     5u32
 }
@@ -56,6 +59,7 @@ use std::fmt::Debug;
 
 type Foo<T: Debug> = impl Debug;
 
+#[define_opaque(Foo)]
 fn foo<U: Debug>() -> Foo<U> {
     Vec::<U>::new()
 }