Move mixed export_name/no_mangle check to check_attr.rs and improve the error

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_passes/src/errors.rs

@@ -49,6 +49,18 @@ pub(crate) struct ConstContinueAttr {
     pub node_span: Span,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(passes_mixed_export_name_and_no_mangle)]
+pub(crate) struct MixedExportNameAndNoMangle {
+    #[label]
+    #[suggestion(style = "verbose", code = "", applicability = "machine-applicable")]
+    pub no_mangle_span: Span,
+    #[note]
+    pub export_name_span: Span,
+    pub no_mangle_attr: &'static str,
+    pub export_name_attr: &'static str,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(passes_outer_crate_level_attr)]
 pub(crate) struct OuterCrateLevelAttr;