This avoids throwing ExecutionEngineException-s if one of the public APIs is called on a non-allocated DependentHandle instance

The ephemeron type is checked in the Mono runtime in "object.c" as follows:

m_class_get_image (klass) == mono_defaults.corlib && !strcmp ("Ephemeron", m_class_get_name (klass))

As such, the namespace it belongs to in the managed runtime doesn't matter: the VM will just check
that the type name matches, and that the type is in fact defined in corelib. This means we can
just move it to System.Runtime without worrying about it being properly managed in the VM.
Additionally, the type is defined in "sgen-mono.c" as follows:

typedef struct {
    GCObject* key;
    GCObject* value;
} Ephemeron;

So as long as the layout matches the one of the type defined in C# (which it does), we're also free
to rename the fields to better follow the naming guidelines, and the VM will have no issues with it.

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Removed internal call, same optimization as GCHandle

This fixes a bug due to a race condition in ConditionalWeakTable<K, V>, which relies on this method which atomically retrieves both target and dependent with respect to target being set to null concurrently by other threads. This also exposes the same API publically to allow consumers to potentially implement custom conditional weak tables in the same manner.

This also fixes two potential GC holes when setting DependentHandle.Target (see conversation from dotnet#54246 (comment) onwards)

This was added in dotnet#39810 to avoid a collision with MarshalNative::GCHandleInternalSet, as the two FCalls had identical implementations and their entry points were not unique. This should no longer be needed after 099fc478551f46cc54e7a18a32d9a9ac73727c73, as that changed both the signature and the implementation of this FCall.