fix: eager resolution of unused type arguments

Fixes #881.

spec/lang/subtyping/typealias_spec.lua

@@ -32,4 +32,15 @@ describe("typealias", function()
          end
       end
    ]]))
+
+   it("resolves early even if type arguments are not used (regression test for #881)", util.check([[
+      local record Foo<T> end
+      local record Bar<T> end
+
+      local type StringFoo = Foo<string>
+      local type StringBar = Bar<string>
+
+      local type Test = {StringFoo: StringBar}
+      local test: Test = {}
+   ]]))
 end)

tl.lua

@@ -7649,6 +7649,12 @@ do
                   copy.constraint, same = resolve(t.constraint, same)
                end
             end
+
+
+            local rt = fn_var(self, a_type(t, "typevar", { typevar = t.typearg }))
+            if rt then
+               resolved[t.typearg] = rt
+            end
          elseif t.typename == "unresolvable_typearg" then
             assert(copy.typename == "unresolvable_typearg")
             copy.typearg = t.typearg

tl.tl

@@ -7649,6 +7649,12 @@ do
                   copy.constraint, same = resolve(t.constraint, same)
                end
             end
+
+            -- eager resolution of type argument variables
+            local rt = fn_var(self, a_type(t, "typevar", { typevar = t.typearg } as TypeVarType))
+            if rt then
+               resolved[t.typearg] = rt
+            end
          elseif t is UnresolvableTypeArgType then
             assert(copy is UnresolvableTypeArgType)
             copy.typearg = t.typearg