…hich aren't provably immutable.

This is safe, as any instantiated object must have immutable type parameters.

Note: this is only true for `@ImmutableTypeParameter`, not `containerOf`! The latter is conditionally immutable, so we really do need to check the bound.

(And the corresponding change for ThreadSafe.)

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…parameters are subtypes of another overload.

JLS defines that more specific overloads win, so, for example, `ImmutableFunction` wins over `Function`.

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…n JDK 11, and prepare to add test coverage for that configuration

Thanks to Stephan202's suggestion in #3895 (comment)

#3895

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