[clang-format] [JS] handle private members.

Addresses PR40999 https://bugs.llvm.org/show_bug.cgi?id=40999

Private fields and methods in JavaScript would get incorrectly indented
(it sees them as preprocessor directives and hence left aligns them)

In this revision `#identifier` tokens `tok::hash->tok::identifier` are
merged into a single new token `tok::identifier` with the `#` contained
inside the TokenText.

Before:

```
class Example {
  pub = 1;

  static pub2 = "foo";
  static #priv2 = "bar";

  method() { this.#priv = 5; }

  static staticMethod() {
    switch (this.#priv) {
    case '1':
      break;
    }
  }

  this.#privateMethod(); // infinite loop
}

static #staticPrivateMethod() {}
}
```

After this fix the code will be correctly indented

```
class Example {
  pub = 1;
  #priv = 2;

  static pub2 = "foo";
  static #priv2 = "bar";

  method() { this.#priv = 5; }

  static staticMethod() {
    switch (this.#priv) {
    case '1':
      #priv = 3;
      break;
    }
  }

  #privateMethod() {
    this.#privateMethod(); // infinite loop
  }

  static #staticPrivateMethod() {}
}
```

NOTE: There might be some JavaScript code out there which uses the C
processor to preprocess .js files
http://www.nongnu.org/espresso/js-cpp.html. It's not clear how this
revision or even private fields and methods would interact.

Patch originally by MyDeveloperDays (thanks!).

llvm-svn: 356449