support for external types with covariant generic params

tests/errmsgs/tinvalidinout.nim

@@ -0,0 +1,26 @@
+discard """
+cmd: "nim check $file"
+errormsg: "`in` and `out` flags can be used only with imported types"
+nimout: '''
+tinvalidinout.nim(14, 7) Error: `in` and `out` flags can be used only with imported types
+tinvalidinout.nim(17, 9) Error: `in` and `out` flags can be used only with imported types
+tinvalidinout.nim(18, 9) Error: `in` and `out` flags can be used only with imported types
+'''
+"""
+
+type
+  Foo {.header: "foo.h", importcpp.} [in T] = object
+
+  Bar[out X] = object
+    x: int
+
+proc f1[in T](x: T) = discard
+proc f2[in T](x: T) {.importc: "f", header: "foo.h"}
+
+var
+  f: Foo[int]
+  b: Bar[string]
+
+f1 f
+f2 b
+