Expose Resolve helper for assemblies (dotnet/linker#2074)

* Implement IAssemblyResolver for LinkContext

Fixes dotnet/linker#2073

* Fix using order

* Don't implement IAssemblyResolver

Commit migrated from dotnet/linker@802aa45

src/tools/illink/src/linker/Linker/LinkContext.cs

@@ -307,6 +307,11 @@ public AssemblyDefinition Resolve (IMetadataScope scope)
 			return _resolver.Resolve (reference);
 		}
 
+		public AssemblyDefinition Resolve (AssemblyNameReference name)
+		{
+			return _resolver.Resolve (name);
+		}
+
 		public void RegisterAssembly (AssemblyDefinition assembly)
 		{
 			if (SeenFirstTime (assembly)) {

src/tools/illink/src/linker/ref/Linker/LinkContext.cs

@@ -28,5 +28,7 @@ internal LinkContext () { }
 		public MethodDefinition TryResolve (MethodReference methodReference) { throw null; }
 		public FieldDefinition TryResolve (FieldReference fieldReference) { throw null; }
 		public TypeDefinition TryResolve (TypeReference typeReference) { throw null; }
+
+		public AssemblyDefinition Resolve (AssemblyNameReference nameReference) { throw null; }
 	}
 }