Readonly Type fields should be treated like Type variables

In trying to make the following code trim compatible:

https://github.com/dotnet/runtime/blob/1a1ad943288b512a877222e2fdede0bc198df333/src/libraries/System.Private.Xml/src/System/Xml/Xsl/IlGen/GenerateHelper.cs#L81-L103

I wanted to write the following code:

```C#
internal class XmlILStorageMethods
{
public readonly Type SeqType;
public readonly MethodInfo SeqAdd;

public XmlILStorageMethods(Type storageType)
{
if (storageType == typeof(XPathNavigator))
{
SeqType = typeof(XmlQueryNodeSequence);
SeqAdd = SeqType.GetMethod("AddClone")!;
}
else if (storageType == typeof(XPathItem))
{
SeqType= typeof(XmlQueryItemSequence);
SeqAdd = SeqType.GetMethod("AddClone")!;
}
else
{
SeqType = typeof(XmlQuerySequence<>).MakeGenericType(storageType);
SeqAdd = SeqType.GetMethod("Add")!;
}

FieldInfo? seqEmpty = SeqType.GetField("Empty");
}
}
```

However, the linker warns here that I haven't annotated the `Type SeqType` field appropriately on the calls to `.GetMethod` and that final line `SeqType.GetField("Empty");`. But in this case, the field is readonly, and it is unconditionally set above the usage.

```
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(99,17): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String)'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(116,13): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String,Type[])'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(115,13): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String,Type[])'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(104,17): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String)'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(109,17): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String)'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(112,13): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetField(String)'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
F:\git\runtime\src\libraries\System.Private.Xml\src\System\Xml\Xsl\IlGen\GenerateHelper.cs(117,13): Trim analysis warning IL2080: System.Xml.Xsl.IlGen.XmlILStorageMethods.XmlILStorageMethods(Type): The requirements declared via the 'DynamicallyAccessedMembersAttribute' on the field 'System.Type System.Xml.Xsl.IlGen.XmlILStorageMethods::SeqType' don't match those on the implicit 'this' parameter of method 'System.Type.GetMethod(String)'. The source value must declare at least the same requirements as those declared on the target location it's assigned to [F:\git\runtime\src\libraries\src.proj]
```

To work around the linker, I am forced to introduce an intermediate variable for `Type sequenceType`, and use that to call `.GetMethod` and `.GetField("Empty")`. Then set the field to the variable at the end.

In this case, since the field is `readonly` and it is unconditionally set to a concrete Type above, I shouldn't need to be forced to inject a local variable just to make the linker happy.