DecompilerTypeSystem uses this to resolve/load multiple assemblies in parallel.
Unfortunately this doesn't gain us any performance yet in ILSpy because there we have a global assembly-loader-lock :(
We now avoid the old `IModuleReference` interface which required allocating for every type being resolved.
Instead `MetadataModule.ResolveModule` now combines decoding+resolving assembly references into a single step.
This allows the type system to maintain a cache indexed by row number.
This also changes the behavior of resolving references within a compilation: We now prefer an exact match (name + version + publickeytoken) first; and fall back to a name-only match only if no exact match exists.
This somewhat improves the decompilation of assemblies created by using ilmerge to combine assemblies with different target frameworks.
This initial commit only handles the trivial case where an Index or Range object is constructed.
The TODO portions of the test case show there are plenty of cases where where the C# compiler emits more complex code patterns that will require ILAst transforms.
If this attribute is in use, private/internal members lack nullability annotations.
Previously in such cases, we ended up inheriting the nullability from the `[NullableContext]`, which could cause us to display a misleading nullability for primary methods.
In debug builds, it could also trigger an assertion when trying to apply the "nullable reference type" marking to to value types.
Of note is that properties and events are a special case: they do not explicitly store Accessibility in metadata. For properties computing the accessibility requires decoding the signature (to find overridden base properties). So these two only check the declaring type's accessibility instead; private properties may still carry nullability despite `[NullablePublicOnly]`. However, the property accessors won't store nullability, so we need to read the `returnTypeAttributes` from the property itself.
* rename a few identifiers,
* add comments,
* skip all compiler-generated type-arguments in resolve result,
* and change implementation of LocalFunctionMethod.Specialize: wrap specialized method and not the other way round.