AST: Revert disambiguation of overloads by obsoletion version.
It caused a regression when building the CloudKit module from its interface in
the Xcode 26 SDKs.
This effectively reverts https://github.com/swiftlang/swift/pull/83384 (though
a few parts of that PR that did not cause any regressions have been kept).
Resolves rdar://157342004.
