sess: add create_{err,warning}

Currently, the only API for creating errors from a diagnostic derive
will emit it immediately. This makes it difficult to add subdiagnostics
to diagnostics from the derive, so add `create_{err,warning}` functions
that return the diagnostic without emitting it.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_session/src/session.rs

@@ -413,9 +413,21 @@ impl Session {
     pub fn err(&self, msg: impl Into<DiagnosticMessage>) -> ErrorGuaranteed {
         self.diagnostic().err(msg)
     }
+    pub fn create_err<'a>(
+        &'a self,
+        err: impl SessionDiagnostic<'a>,
+    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+        self.parse_sess.create_err(err)
+    }
     pub fn emit_err<'a>(&'a self, err: impl SessionDiagnostic<'a>) -> ErrorGuaranteed {
         self.parse_sess.emit_err(err)
     }
+    pub fn create_warning<'a>(
+        &'a self,
+        err: impl SessionDiagnostic<'a, ()>,
+    ) -> DiagnosticBuilder<'a, ()> {
+        self.parse_sess.create_warning(err)
+    }
     pub fn emit_warning<'a>(&'a self, warning: impl SessionDiagnostic<'a, ()>) {
         self.parse_sess.emit_warning(warning)
     }