improve diagnostics on top_retain & mangle.properties (#5622)

closes #5618

test/compress/issue-1770.js

@@ -53,6 +53,14 @@ mangle_props: {
         );
     }
     expect_stdout: "1 1 1 2 2 2 3 3 3 4 4 4 5 5"
+    expect_warnings: [
+        "INFO: Preserving reserved property undefined",
+        "INFO: Preserving reserved property NaN",
+        "INFO: Preserving reserved property Infinity",
+        "INFO: Preserving reserved property -Infinity",
+        "INFO: Preserving reserved property null",
+        "INFO: Preserving reserved property log",
+    ]
 }
 
 numeric_literal: {
@@ -106,6 +114,11 @@ numeric_literal: {
         "4 5 4 4",
         "8 7 8",
     ]
+    expect_warnings: [
+        "INFO: Preserving reserved property log",
+        "INFO: Mapping property 0x25 to o",
+        "INFO: Mapping property 1E42 to b",
+    ]
 }
 
 identifier: {