Track FieldBuilder volatility

[nasser]

Due to a limitation in System.Reflection.Emit we cannot call GetRequiredCustomModifiers on FieldBuilders. We need this to know if a FieldBuilder is volatile or not. We were doing our own bookkeeping to keep track of this, but a recent change has rolled that back. We might want to look into a more generic strategy to get around System.Refection.Emit limitations, because this keeps coming up...