Fix dynamic type index calculation in ApplyAttributeTypeVisitor

[ElektroKill]

Link to issue(s) this covers:
N/A

Problem

ILSpy did not correctly decompile volatile dynamic fields.

Example:

static volatile dynamic volatileField;

would decompile to:

static volatile object volatileField;

The issue lies in the implementation of ApplyAttributeTypeVisitor. This class did not properly handle modifier signatures (CModOpt and CModReqd) which are used to indicate that a field is volatile.

Roslyn implementation: https://github.com/dotnet/roslyn/blob/29cdbf9a70ae83e6b7b914182fc78a7ed122a0a0/src/Compilers/CSharp/Portable/Symbols/Compilation_WellKnownMembers.cs#L850

Solution

Modifier signatures are now correctly handled. The dynamicTypeIndex is incremented for every modifier type to match Roslyn's behavior when it generates the DynamicAttribute attributes. The "crazy special case" for readonly return types has been removed as those include a modifier signature of InAttribute.

The dynamic "pretty" test was extended to include a dynamic volatile field.

[dgrunwald]

We need to get rid of the ModifiedType, otherwise it might confuse other parts of the type system that don't expect it to be present (e.g. in overload resolution).
But I think you can re-order the ApplyAttributesToType call to come before this if.

[ElektroKill]

Doesn't ApplyAttributesToType already remove modifier types? Or is that option not enabled by default? It seems that other places using ApplyAttributesToType where modifier types might appear do not have some special handling for them (eg. method return types).

[dgrunwald]

Ah right, I forgot that ApplyAttributesToType usually removed modopts.
Pretty much only the ILSpy UI uses TypeSystemOptions.KeepModifiers to keep them around, and in that case it seems harmless to also keep IsVolatile around.