[stdlib] Rearrange + on RangeReplaceableCollection/Sequence once again

[moiseev]

Based on the discssion here #13697

[moiseev]

@swift-ci Please smoke test

[moiseev]

/cc @jrose-apple and @xedin

[jrose-apple]

I'm still confused as to why we can't put the other one on RRC too, but sure.

(i.e. if there's an ambiguity error, that'd be a type-checker bug, right?)

[moiseev]

@jrose-apple The code that gets ambiguous is this: [first] + rest. Where rest is an Array. But, [first] is not strictly an array, it can also be a Set, which is not a RRC. Therefore, I think, ambiguity is between "set as Sequence + array as RRC" and "array as RRC + array as RRC".

[jrose-apple]

Hm. How is rest + [last] not equally ambiguous, then?

We won't pull the type "Set" out of thin air, and we should still default to Array as the default array literal type. So I'm not sure I buy this explanation.

What does -Xfrontend -debug-constraints say? cc @xedin

[moiseev]

Hm. How is rest + [last] not equally ambiguous, then?

Maybe it is, but we don't have expression like this in the codebase.

[xedin]

@jrose-apple Looked it for a bit this morning, the other way around is also ambiguous in stdlib :)

swift/stdlib/public/core/Codable.swift:2686:78: error: ambiguous use of operator '+'
        let context = DecodingError.Context(codingPath: container.codingPath + [key],
                                                                             ^

[moiseev]

After having discussed with @xedin we decided to merge this PR and then, when the ranking bug is fixed, move the last + overload from Sequence to RangeReplaceableCollection.