[4.0][stdlib] Fix array slice self-assignment bug

[airspeedswift]

Cherry pick of #10958 onto the 4.0 branch.

• Explanation: When fast-pathing in-place slice mutation, bounds check the slice's ranges match exactly. In-place mutation of an array slice doesn't need assigning back, but there needs to be a check that the assigned slice's bounds matches otherwise unmatched range assignments like a[1..<1] = a[1..<2] were getting thrown away.
• Scope of Issue: Correctness fix
• Risk: Minimal. This is checking for a potential no-op fast path. When the check fails, the full replaceRange method is called, writing the slice into the collection in full. This is what is desired in this case.
• Reviewed By: Dave Abrahams
• Testing: Automated test suite + new tests for this case