Remove defensive copies in System.Reflection.MetadataLoadContext

[hamarb123]

These fields being marked readonly causes 25 uses to create a defensive copy in total (since it's generic, it has no such things as readonly members, meaning every access requires a defensive copy). Removing the readonly should resolve these.

[stephentoub]

Can you highlight specific examples where this is causing meaurably worse performance / worse code generation?

[hamarb123]

Can you highlight specific examples where this is causing meaurably worse performance / worse code generation?

Looking at the definition of EcmaMethodDecoder : IMethodDecoder, it seems like it'd be 4 pointer sizes. So the longer methods that will not be inlined will copy 32 bytes first before they can call the method (on 64-bit). I can do some proper testing at some point, but I'll probably be unable to do much for approximately the next 1.5 weeks.

[hamarb123]

Can you highlight specific examples where this is causing meaurably worse performance / worse code generation?

This is probably not that bad really, so I'll close it, since the preference is to keep readonly where possible without meaningful performance difference (#101156 (comment)).

[stephentoub]

Thanks