[SyntaxTree] Unbox operators into tokens for nodes generated from `CXXOperatorCallExpr`

For an user define `<`, `x < y` would yield the syntax tree:
```
BinaryOperatorExpression
|-IdExpression
| `-UnqualifiedId
|   `-x
|-IdExpression
| `-UnqualifiedId
|   `-<
`-IdExpression
  `-UnqualifiedId
    `-y
```
But there is no syntatic difference at call site between call site or
built-in `<`. As such they should generate the same syntax tree, namely:
```
BinaryOperatorExpression
|-IdExpression
| `-UnqualifiedId
|   `-x
|-<
`-IdExpression
  `-UnqualifiedId
    `-y
```

Differential Revision: https://reviews.llvm.org/D85750