Higher order type relations for mapped types

[ahejlsberg]

This PR adds a number of type relations for higher order mapped types and indexed access types (i.e. un-instantiated generic forms of the types).

• A type S[K] is related to a type T[K] if S is related to T.
• A type T is related to a type { [P in keyof T]: X } if T[P] is related to X.
• A type { [P in keyof T]: X } is related to a type T if X is related to T[P].

The PR furthermore adds the following rules:

• The operation keyof { [P in K]: X } is equivalent to just K. For example, keyof Partial<T> is equivalent to keyof T.
• The operation { [P in K]: T}[X] is equivalent to an instantiation of T where X is substituted for every occurrence of P. For example, { [P in K]: Box<T[P]> }[X] is equivalent to Box<T[X]>.
• In type T = { [P in K]: X } it is an error for K to directly or indirectly reference T.
• Given a type parameter T with a constraint that includes a string index signature of type X, the apparent type of an indexed access type T[K] is X.

Fixes #12311.
Fixes #12315.