[sgen] Ensure minor_collection_allowance is always initialized. This avoid a spurious GC on startup.
[System.Runtime.Serialization] Fix from Brendan Zagaeski for bug #17663
Fix memory leak.

Introduced in b322a6e41038f7bf35f8a433b34c3b18e4a6d169. variance_used
was not being set before use, resulting in leaked trampolines when
calling methods on variant interfaces.