sidekick: suppress swift warnings for specific functions

Starting with Swift 6.4 we should be able to suppress warnings in some specific functions:

https://github.com/swiftlang/swift-evolution/blob/main/proposals/0522-source-warning-control.md

This is desirable because:

- We should use `-warnings-as-errors` in CI and development to detect portability and future-proof problems as early as possible
- Note that we must not force `-warnings-as-errors` on customers, because they may be compiling with a different toolchain
- We should to emit deprecation attributes for deprecated elements, so customers get a warning when they use such elements
- We must use these deprecated elements in the generated code, to support them if they are still used by the application or received from the wire

If we enable `-warnings-as-errors` in our builds then any generated code using the deprecated elements fails to build. We are forced to use `-warnings-as-errors` **and** `-suppress-warnings DeprecatedDeclaration` which leaves us blind to using deprecated things in hand-crafted code.

Start by reading Swift Evolution proposal SE-0522 and inspect the current use of -warnings-as-errors and -suppress-warnings DeprecatedDeclaration in the build and generated-code paths. Done means generated code using deprecated elements still compiles while deprecation warnings remain visible for hand-written code.

Written by the indexing model from the issue text.