SR-5810: Fix slice data range calculation when inserting/deleting an element

[sashabelonogov]

I recreated the bug described in the link below with 2 tests:
https://bugs.swift.org/browse/SR-5810

I found an issue in Data.init(backing: _DataStorage, range: Range) and, it seems like, successfully fixed it.
Please let me know if everything is done properly. Thanks

[alblue]

@swift-ci please test

[alblue]

@phausler @parkera Since Swift 4.0 has now shipped, can we get this merged in and cherry-picked back to the swift-4 updates branch?

[alblue]

Or has swiftlang/swift#11939 rendered this unnecessary?

[ianpartridge]

The fixes in swiftlang/swift#11939 need porting to this repo. Once that's done I suspect this PR will be obsolete although I'm not certain. @phausler would know for sure.

[phausler]

Yea, but I will end up merging the tests once I have my patch up: hopefully later today

[sashabelonogov]

Despite the fact that this PR is obsolete, did I do everything correctly? Is it ok to try to fix bugs from JIRA, assign them to me and send the PR or shouldn't I try this again?

[phausler]

what you did was fine, there was just some underlying issues that were more pervasive than just the issue reported is all. I was working on the other side of it in the overlay and trying to hunt down a good solution for the more general failure.

[sashabelonogov]

thanks @phausler!

[phausler]

btw I am still going to merge your test (with some modifications) since it is still valid

[sashabelonogov]

Should I reopen the PR with the tests only?

[phausler]

if you would like, the only alteration is that the starting indexes dont change after mutation now; it follows the same behavior as Array and ArraySlice indexing.

so in your first test the startIndex after mutation is 2 not 0 etc...

[sashabelonogov]

Makes sense, I will make it now

[sashabelonogov]

@phausler I updated the tests and created a new PR with tests only #1235