Issue Description:

Straight out of Sequence.swift, from comment for optimization to avoid multiple wrappers when chaining dropFirst calls:

[1,2,3].dropFirst(1).dropFirst(1)

produces following error:

DropFirstChainOnArrayAmbiguous.swift:1:1: error: ambiguous use of 'dropFirst'
[1,2,3].dropFirst(1).dropFirst(1)
^
Swift.Collection:39:17: note: found this candidate
    public func dropFirst(_ n: Int) -> Self.SubSequence
                ^
Swift.Sequence:20:17: note: found this candidate
    public func dropFirst(_ n: Int) -> AnySequence<Self.Iterator.Element>
                ^

Workaround is to split the expression in two, storing first part in variable as seen in the tests for that method:

 let result0 = [1,2,3].dropFirst(1)
 let result1 = result0.dropFirst(1)

Given that we are just storing subexpressions in variables, without having to explicitly define types, this is a bug in compiler, which should be able to infer the most specific type - Collection in this case.