Intersection types are not valid index signature types

[weswigham]

For example, a variable of type

type Path = string & { __pathBrand: any };

cannot be used as an indexer type, and cannot be used to index an object with a string type indexer (despite being safely usable as a string in all other locations).

If an intersection type contains types which are valid for use in an index signature, then the intersection type itself should probably be valid as an index signature.

[ivogabe]

Looks like a duplicate of #4372