[prebuilt_code_assets] Distinguish disabled record use from no used functions

The docs and messages said "no recorded uses" for two different cases:

- Record use is disabled (`input.recordedUses` is `null`): it is unknown
  which functions the application uses. They now say "record use is
  disabled".
- The application uses none of the functions: `link` now explicitly
  bundles no library and returns before setting up the linker, instead of
  relying on `CLinker` skipping an empty `symbolsToKeep`. This also holds
  with `treeshake: on`.

Adds unit tests for the second case on Linux and Windows in the `auto` and
`on` modes, and an integration test that a `dart build cli` app that calls
none of the functions runs without the library.