[vscode-graphql] Auto-complete doesn't work when string is not assigned to variable

This might be related to #2270? I can reliably reproduce the following. I'm using the `/* GraphQL */` comment prefixing a normal template literal (not using `gql` tag). If this string is assigned to a variable, the auto-complete works, if not then it won't. Either way the syntax highlighting will be applied, but only in the first case does it actually provide the auto-complete.

Working:
```
const anything = /* GraphQL */ `
...etc
`
```

Not working:
```
/* GraphQL */ `
...etc
`
```

Note this doesn't occur when using the `gql` tag in the same way. I haven't been able to track this down as to why that is the case.

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For context, my GraphQL strings are being processed by an external tool and code is generated from them, but they are never used in the app directly. So it is helpful to not assign them to variables to make it clear they are never used (and shouldn't be used).