[SR-13965] Warn on shadowed type parameters

[typesanitizer]

Previous ID	SR-13965
Radar	rdar://problem/45735133
Original Reporter	@typesanitizer
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, TypeChecker
Assignee	None
Priority	Medium

md5: 5db23a1c00e99b556cb85597f6d03045

Issue Description:

We allow shadowed type parameters, which can lead to confusing diagnostics.

struct S<T> {
    var t: T
    init<T>(_ myT : T) {
        t = myT // error: cannot assign value of type 'T' to type 'T'
    }
}

In this case, there are two rigid type variables, one for the outer parameter and one for the inner parameter, but since they are both rigid, they can't be equal.

Emitting a warning would guide the programmer to simplifying their code so that we can produce a better diagnostic.