Fix #13093 C++ Atomics: operator= is implicitly deleted because the default definition would be ill-formed (#21169)

* add test

* fix #17982 Invalid C++ code generation when returning discardable var T

* fix #13093

* cpp atomic good example

* clearify the condition

tests/cpp/t13093.nim

@@ -0,0 +1,24 @@
+discard """
+  targets: "cpp"
+  action: reject
+  errormsg: "The PledgeObj type requires the following fields to be initialized: refCount"
+"""
+
+import atomics
+
+type
+  Pledge* = object
+    p: PledgePtr
+
+  PledgePtr = ptr PledgeObj
+  PledgeObj = object
+    refCount: Atomic[int32]
+
+proc main() =
+  var pledge: Pledge
+  pledge.p = createShared(PledgeObj)
+  let tmp = PledgeObj() # <---- not allowed: atomics are not copyable
+
+  pledge.p[] = tmp
+
+main()

tests/cpp/t17982.nim

@@ -0,0 +1,20 @@
+discard """
+  targets: "cpp"
+  action: "compile"
+"""
+
+type
+  String* {.importcpp: "std::string", header: "string".} = object
+
+proc initString*(): String
+    {.importcpp: "std::string()", header: "string".}
+
+proc append*(this: var String, str: String): String
+    # bug seems to trigger when `#`, `@`, or `$1` is used inside `importcpp`
+    {.importcpp: "#.append(@)", header: "string", discardable.} # <- changed from `importcpp: "append"`
+
+var
+  s1 = initString()
+  s2 = initString()
+
+s1.append s2