[Mono] Support invoking abstract constructors on existing instances through MethodInvoker

[jkoritzinsky]

Description

The reflection invocation tests for abstract constructors invoked on an existing derived instance are currently limited to CoreCLR because Mono does not support the behavior exercised by MethodInvokerTests.Constructor_AbstractDeclaringTypeWithExistingInstance, Constructor_AbstractDeclaringTypeWithExistingInstance_RegularArguments, and Constructor_AbstractDeclaringTypeWithExistingInstance_ManyRegularArguments.

Mono should support invoking an instance constructor declared on an abstract base type against an existing compatible derived object, including both regular and by-ref argument paths.

Reproduction Steps

Run the System.Reflection.Tests tests listed above on Mono. The tests are currently marked ConditionalFact(IsCoreCLR) and should become active on Mono once the behavior is implemented.

Representative pattern:

ConstructorInfo constructor = typeof(AbstractRegularConstructorTarget).GetConstructor(
    BindingFlags.Instance | BindingFlags.NonPublic,
    binder: null,
    new[] { typeof(int) },
    modifiers: null)!;
MethodInvoker invoker = MethodInvoker.Create(constructor);
var target = new ConcreteRegularConstructorTarget(0);
invoker.Invoke(target, 42);

Expected behavior

The constructor runs against the existing derived instance and updates its state, as it does on CoreCLR. Allocating an instance for an abstract declaring type and invoking with a null target should remain rejected.

Actual behavior

The behavior is not currently supported on Mono, so these tests cannot run there and are guarded to CoreCLR.

Regression?

Unknown.

Known Workarounds

None.

Configuration

Mono runtime; System.Reflection.Tests.

Other information

Relevant tests: src/libraries/System.Runtime/tests/System.Reflection.Tests/MethodInvokerTests.cs.

[!NOTE] This issue was filed with assistance from GitHub Copilot.

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