in keyword doesn't narrow types with string-indexers correctly

[sandersn]

This is broken in the new narrowing for the in keyword (#15256).
The test inKeywordTypeguard needs to add the following test:

interface StringMap<T> {
  [key: string]: T
}
function f(options: StringMap<any>) {
  if ('data' in options) {
    return options['data'];
  }
}

Right now there is an error on options['data'], but there shouldn't be.

[sandersn]

Fix is up at #20543