Add --mangle-props-debug and fix --mangle-props=unquoted collision

Patch by @AshleyScirra

Based on: PR #1316

Renamed the CLI debug option to --mangle-props-debug

Fixes: #1321 name collision in --mangle-props=unquoted

test/compress/issue-1321.js

@@ -0,0 +1,54 @@
+issue_1321_no_debug: {
+    mangle_props = {
+        ignore_quoted: true
+    }
+    input: {
+        var x = {};
+        x.foo = 1;
+        x["a"] = 2 * x.foo;
+        console.log(x.foo, x["a"]);
+    }
+    expect: {
+        var x = {};
+        x.b = 1;
+        x["a"] = 2 * x.b;
+        console.log(x.b, x["a"]);
+    }
+}
+
+issue_1321_debug: {
+    mangle_props = {
+        ignore_quoted: true,
+        debug: ""
+    }
+    input: {
+        var x = {};
+        x.foo = 1;
+        x["_$foo$_"] = 2 * x.foo;
+        console.log(x.foo, x["_$foo$_"]);
+    }
+    expect: {
+        var x = {};
+        x.a = 1;
+        x["_$foo$_"] = 2 * x.a;
+        console.log(x.a, x["_$foo$_"]);
+    }
+}
+
+issue_1321_with_quoted: {
+    mangle_props = {
+        ignore_quoted: false
+    }
+    input: {
+        var x = {};
+        x.foo = 1;
+        x["a"] = 2 * x.foo;
+        console.log(x.foo, x["a"]);
+    }
+    expect: {
+        var x = {};
+        x.a = 1;
+        x["b"] = 2 * x.a;
+        console.log(x.a, x["b"]);
+    }
+}