Spec needs to account for computed constant names

[DanielRosenwasser]

• https://github.com/Microsoft/TypeScript/blob/4b96edf72faf6f9b6b464e148c9c02eac96aa4bd/doc/spec.md#22-names

  a ComputedPropertyName that denotes a well-known symbol (2.2.3).

• https://github.com/Microsoft/TypeScript/blob/b8f22f51441495203c96b95969c45a3f78482517/doc/spec.md#391-property-signatures

This should be updated once #15473 goes in