[1.0] Freeze some types for consistency with their inlinable initializers

[lorentey]

Note: @frozen does not mean that we actually consider these struct types frozen. Their layout may arbitrarily change between even minor releases.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test