[clang-format] Remove spurious JSON binding when DisableFormat = true

Relevant issue: https://github.com/llvm/llvm-project/issues/52705

When the `DisableFormat` option of `clang-format` is set to `true` and a JSON file is formatted, the ephemeral variable binding that is added to the top-level object is not removed from the formatted file.  For example, this JSON:
```
{
  "key": "value"
}
```
Is reformatted to:
```
x = {
  "key": "value"
}
```
Which is not valid JSON syntax.  This fix avoids the addition of this binding when `DisableFormat` is set to `true`, ensuring that it cannot be left behind when formatting is disabled.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D115769

Fixes #52705