Don't retain modifiers for instantation stub signatures (dotnet/coreclr#23210)

Don't retain modifiers for instantation stub signatures (dotnet/coreclr#23210)

`ConvToJitSig` is not expecting to see modifiers for return types in sigs,
so don't bother preserving them when creating instantiation stubs.

This comes up for instantiation stubs for methods of `ReadOnlySpan<T>`.

Also, if we are preserving modifiers in a sig, make sure to prefix their type
handles with `ELEM_TYPE_INTERNAL`.

Fixes dotnet/coreclr#23136.

Commit migrated from https://github.com/dotnet/coreclr/commit/5831fe40394fcdc34fa161041956d379db433dd6