Fix some out-of-date comments.

Author: nnethercote

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1004,9 +1004,9 @@ pub(crate) enum Message<B: WriteBackendMethods> {
 /// process another codegen unit.
 pub struct CguMessage;
 
-// A cut-down version of `rustc_errors::Diagnostic` that impls `Send`, which
+// A cut-down version of `rustc_errors::DiagInner` that impls `Send`, which
 // can be used to send diagnostics from codegen threads to the main thread.
-// It's missing the following fields from `rustc_errors::Diagnostic`.
+// It's missing the following fields from `rustc_errors::DiagInner`.
 // - `span`: it doesn't impl `Send`.
 // - `suggestions`: it doesn't impl `Send`, and isn't used for codegen
 //   diagnostics.
@@ -1021,8 +1021,8 @@ struct Diagnostic {
     args: DiagArgMap,
 }
 
-// A cut-down version of `rustc_errors::SubDiagnostic` that impls `Send`. It's
-// missing the following fields from `rustc_errors::SubDiagnostic`.
+// A cut-down version of `rustc_errors::Subdiag` that impls `Send`. It's
+// missing the following fields from `rustc_errors::Subdiag`.
 // - `span`: it doesn't impl `Send`.
 pub struct Subdiagnostic {
     level: Level,