[embedded] Add support for ManagedBuffer to Embedded Swift

[kubamracek]

This makes ManagedBuffer available and usable in Embedded Swift, by:

• Removing an internal consistency check from ManagedBuffer that relies on metatypes.
• Making the .create() API transparent (to hoist the metatype to the callee).
• Adding a AllocRefDynamicInst simplification to convert alloc_ref_dynamic to alloc_ref, which removes a metatype use.
• Adding tests for the above.