provide diagnostic details on --mangle-props (#5621)

closes #5618

lib/propmangle.js

@@ -271,7 +271,10 @@ function mangle_properties(ast, options) {
     }
 
     function mangle(name) {
-        if (!should_mangle(name)) return name;
+        if (!should_mangle(name)) {
+            AST_Node.info("Preserving property {this}", name);
+            return name;
+        }
         var mangled = cache.get(name);
         if (!mangled) {
             if (debug) {
@@ -286,6 +289,10 @@ function mangle_properties(ast, options) {
             if (/^#/.test(name)) mangled = "#" + mangled;
             cache.set(name, mangled);
         }
+        AST_Node.info("Mapping property {name} to {mangled}", {
+            mangled: mangled,
+            name: name,
+        });
         return mangled;
     }