Pervasive replacement of nkRecWhen in generic types

Long story short, even if the type contains no reference at all to its
generic parameters we still have to walk its AST and evaluate any
nkRecWhen nodes that semRecordNodeAux skipped due to the type being a
generic one.

We also must be careful to modify the type `n` node in place since it
may be referenced by the caller as seen in the tillegaltyperecursion
test.

Moreover we also can't have the nkSym drift away from their original
values in order not to break the JS nkObjConstr codegen.

tests/objects/twhen1.nim

@@ -0,0 +1,51 @@
+const Z = 0
+
+type
+  Foo[T] = object
+   when true:
+     u: int
+   else:
+     v: int
+  Foo1[T] = object
+   when T is int:
+     x: T
+   elif true:
+     z: char
+  Foo2[x:static[int]] = object
+    when (x and 1) == 1:
+      x: array[x+1,int]
+    else:
+      x: array[x,int]
+
+  Foo3 = Foo2[128]
+
+  # #8417
+  Foo4[A: static[int]] = object
+    when Z == 0:
+      discard
+    else:
+      discard
+
+block:
+  var x: Foo[int] = Foo[int](u: 42)
+  doAssert x.u == 42
+
+# Don't evaluate `when` branches before the type is instantiated
+block:
+  var x: Foo1[bool] = Foo1[bool](z: 'o')
+  doAssert x.z == 'o'
+
+block:
+  var x: Foo2[3]
+  doAssert x.x.len == 4
+
+block:
+  var x: Foo2[4]
+  doAssert x.x.len == 4
+
+block:
+  var x: Foo3
+  doAssert x.x.len == 128
+
+block:
+  var x: Foo4[0]