implements RFC: [C++] Constructors as default initializers (#22694)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/cpp/tinitializers.nim

@@ -0,0 +1,33 @@
+discard """
+  targets: "cpp"
+"""
+
+{.emit:"""/*TYPESECTION*/
+struct CppStruct {
+  CppStruct(int x, char* y): x(x), y(y){}
+  void doSomething() {}
+  int x;
+  char* y;
+};
+""".}
+type
+  CppStruct {.importcpp, inheritable.} = object
+  ChildStruct = object of CppStruct
+  HasCppStruct = object
+    cppstruct: CppStruct 
+
+proc constructCppStruct(a:cint = 5, b:cstring = "hello"): CppStruct {.importcpp: "CppStruct(@)", constructor.}
+proc doSomething(this: CppStruct) {.importcpp.}
+proc returnCppStruct(): CppStruct = discard
+proc initChildStruct: ChildStruct = ChildStruct() 
+proc makeChildStruct(): ChildStruct {.constructor:"""ChildStruct(): CppStruct(5, "10")""".} = discard
+proc initHasCppStruct(x: cint): HasCppStruct =
+  HasCppStruct(cppstruct: constructCppStruct(x))
+
+proc main =
+  var hasCppStruct = initHasCppStruct(2) #generates cppstruct = { 10 } inside the struct
+  hasCppStruct.cppstruct.doSomething() 
+  discard returnCppStruct() #generates result = { 10 } 
+  discard initChildStruct() #generates ChildStruct temp ({}) bypassed with makeChildStruct
+  (proc (s:CppStruct) = discard)(CppStruct()) #CppStruct temp ({10})
+main()