Rename some Diagnostic setters.

`Diagnostic` has 40 methods that return `&mut Self` and could be
considered setters. Four of them have a `set_` prefix. This doesn't seem
necessary for a type that implements the builder pattern. This commit
removes the `set_` prefixes on those four methods.

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -70,7 +70,7 @@ fn generic_arg_mismatch_err(
             Res::Err => {
                 add_braces_suggestion(arg, &mut err);
                 return err
-                    .set_primary_message("unresolved item provided when a constant was expected")
+                    .primary_message("unresolved item provided when a constant was expected")
                     .emit();
             }
             Res::Def(DefKind::TyParam, src_def_id) => {