[GR-53803] Only allow Unsafe.allocateInstance for types registered explicitly in the configuration.

[graalvmbot]

This PR tightens the rule Unsafe.allocateInstance in the strict reflection configuration mode (when ThrowMissingRegistrationErrors is enabled): now only types explicitly registered for Unsafe allocation can be allocated that way, i.e., without running a constructor.

The reflection configuration files always had the unsafeAllocated property for types, but we were not enforcing that. Instead, it was possible to allocate all types that the static analysis identified as instantiated. Since there was not a programmatic way to register a type in a Feature, this PR adds registerAsUnsafeAllocated to Feature.BeforeAnalysisAccess.

The JNI method AllocObject does the same as Unsafe.allocateInstance, and is actually implemented exactly like that in SVM. While the preferred way to allocate new instances is to use the JNI function that allocates a new instance and invokes the constructor in the same function, it is also possible to use AllocObject first to allocate an uninitizalized instance, and then via a separate JNI function invoke a constructor on this already existing instance. Since JNI usage is rare, all types that have a constructor registered for JNI are automatically also registered for unsafe allocation. In addition, AllocObject is now tracked by the tracing agent so that such types have the unsafeAllocated property set.

Why tighten the rules? The static analysis can find out which instance fields are never null, and remove unnecessary null checks. But such an optimization is only safe when we know that types are never instantiated without running a constructor, in which case all fields always remain uninitialized. With this new enforcement, the few types explicitly registered for unsafe allocation can easily be excluded from the optimization.

[vvlevchenko]

Is it possible to introduce flag that preserve old behavior for allocation objects via Unsafe at least in debug mode? We use such kind allocation for expression evaluation in plugin for graalvm in intelij ide.