[SR-3979] Collection with two subscripts can't be conformed to RandomAccessCollection without help deducing internal associated types

[jckarter]

Previous ID	SR-3979
Radar	rdar://problem/30567086
Original Reporter	@jckarter
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 7bcf8ad653bc65bcc9e195ec086e49ae

Issue Description:

@gwynne reports that this works in 3.0, but fails in 3.1, unless an explicit typealias _Element is made:

https://gist.github.com/gwynne/80bc43e59a42988133c4c6599ceb5118

[jckarter]

@swift-ci create

[jckarter]

Testing a fix on master: #7564

[jckarter]

And 3.1: #7571