Fix #6415 (#7281)

* hangle static generic params when used in the importcpp pragma

* importcpp works for generic types with fields; fixes #6415

* revert a too agressive safety check that ended up breaking the tests

tests/statictypes/tstaticimportcpp.nim

@@ -0,0 +1,53 @@
+discard """
+targets: "cpp"
+output: "[0, 0, 10, 0]\n5\n1.2\n15\ntest"
+"""
+
+{.emit: """
+
+template <int N, class T>
+struct GenericIntType {
+  T data[N];
+};
+
+template <class T>
+struct GenericTType {
+  T field;
+};
+
+struct SimpleStruct {
+  int field;
+};
+
+
+""" .}
+
+type
+  GenericIntType {.importcpp: "GenericIntType<'0, '1>".} [N: static[int]; T] = object
+    data: array[N, T]
+
+  GenericTType {.importcpp: "GenericTType<'0>".} [T] = object
+    field: T
+
+  GenInt4 = GenericIntType[4, int]
+
+  SimpleStruct {.importcpp: "SimpleStruct"} = object
+    field: int
+
+var
+  a = GenInt4()
+  b = SimpleStruct()
+  c = GenericTType[float]()
+  d = SimpleStruct(field: 15)
+  e = GenericTType[string](field: "test")
+
+a.data[2] = 10
+b.field = 5
+c.field = 1.2
+
+echo a.data
+echo b.field
+echo c.field
+echo d.field
+echo e.field
+