WHITESPACE_ONLY mode does non-whitespace-only optimizations

Closure compiler v20200614

in.js:
```js
{
function foo(p) { self["external1"](p) }

console.log(foo());
console.log(foo());
}
```

Command: `java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level WHITESPACE_ONLY --formatting PRETTY_PRINT`

out.js:
```js
{
var foo = function(p) {
self["external1"](p);
};
console.log(foo());
console.log(foo());
}
;
```

Note that `function foo(p)` was rewritten as `var foo = function(p)`. This is not a whitespace-only change. It also causes a bug (#3623). Therefore even using the lowest tier of WHITESPACE_ONLY optimizations, we cannot escape this bug. I think it is reasonable to expect WHITESPACE_ONLY to only do what it says and not make other adjustments which alter the way scripts work.