Add level arg to into_diagnostic.

And make all hand-written `IntoDiagnostic` impls generic, by using
`DiagnosticBuilder::new(dcx, level, ...)` instead of e.g.
`dcx.struct_err(...)`.

This means the `create_*` functions are the source of the error level.
This change will let us remove `struct_diagnostic`.

Note: `#[rustc_lint_diagnostics]` is added to `DiagnosticBuilder::new`,
it's necessary to pass diagnostics tests now that it's used in
`into_diagnostic` functions.

Author: nnethercote

src/errors.rs

@@ -1,6 +1,6 @@
 use rustc_errors::{
-    DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, ErrorGuaranteed, IntoDiagnostic,
-    IntoDiagnosticArg,
+    DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic,
+    IntoDiagnosticArg, Level,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::Span;
@@ -111,9 +111,13 @@ pub(crate) struct TargetFeatureDisableOrEnable<'a> {
 #[help(codegen_gcc_missing_features)]
 pub(crate) struct MissingFeatures;
 
-impl IntoDiagnostic<'_, ErrorGuaranteed> for TargetFeatureDisableOrEnable<'_> {
-    fn into_diagnostic(self, dcx: &'_ DiagCtxt) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
-        let mut diag = dcx.struct_err(fluent::codegen_gcc_target_feature_disable_or_enable);
+impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for TargetFeatureDisableOrEnable<'_> {
+    fn into_diagnostic(self, dcx: &'_ DiagCtxt, level: Level) -> DiagnosticBuilder<'_, G> {
+        let mut diag = DiagnosticBuilder::new(
+            dcx,
+            level,
+            fluent::codegen_gcc_target_feature_disable_or_enable
+        );
         if let Some(span) = self.span {
             diag.set_span(span);
         };