Cpp Vfunctions draft (#21790)

* introduces virtual pragma, modifies proc def, prevents proc decl

* marks virtual procs as infix

* forward declare vfuncs inside the typedef

* adds naked callConv to virtual

* virtual proc error if not defined in the same top level scope as the type

* first param is now this. extracts genvirtualheaderproc

* WIP syntax

* supports obj. Removes the need for the prefix

* parameter count starts as this. Cleanup

* clean up

* sem tests

* adds integration tests

* uses constraint to store the virtual content

* introduces genVirtualProcParams

---------

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

tests/cpp/tvirtual.nim

@@ -0,0 +1,68 @@
+discard """
+  targets: "cpp"
+  cmd: "nim cpp $file"
+  output: '''
+hello foo
+hello boo
+hello boo
+Const Message: hello world
+NimPrinter: hello world
+NimPrinterConstRef: hello world
+'''
+"""
+
+{.emit:"""/*TYPESECTION*/
+#include <iostream>
+  class CppPrinter {
+  public:
+    
+    virtual void printConst(char* message) const {
+        std::cout << "Const Message: " << message << std::endl;
+    }
+    virtual void printConstRef(char* message, const int& flag) const {
+        std::cout << "Const Ref Message: " << message << std::endl;
+    }  
+};
+""".}
+
+proc newCpp*[T](): ptr T {.importcpp:"new '*0()".}
+type 
+  Foo = object of RootObj
+  FooPtr = ptr Foo
+  Boo = object of Foo
+  BooPtr = ptr Boo
+  CppPrinter {.importcpp, inheritable.} = object
+  NimPrinter {.exportc.} = object of CppPrinter
+
+proc salute(self:FooPtr) {.virtual.} = 
+  echo "hello foo"
+
+proc salute(self:BooPtr) {.virtual.} =
+  echo "hello boo"
+
+let foo = newCpp[Foo]()
+let boo = newCpp[Boo]()
+let booAsFoo = cast[FooPtr](newCpp[Boo]())
+
+#polymorphism works
+foo.salute()
+boo.salute()
+booAsFoo.salute()
+let message = "hello world".cstring
+
+proc printConst(self:CppPrinter, message:cstring) {.importcpp.}
+CppPrinter().printConst(message)
+
+#notice override is optional. 
+#Will make the cpp compiler to fail if not virtual function with the same signature if found in the base type
+proc printConst(self:NimPrinter, message:cstring) {.virtual:"$1('2 #2) const override".} =
+  echo "NimPrinter: " & $message
+
+proc printConstRef(self:NimPrinter, message:cstring, flag:int32) {.virtual:"$1('2 #2, const '3& #3 ) const override".} =
+  echo "NimPrinterConstRef: " & $message
+
+NimPrinter().printConst(message)
+var val : int32 = 10
+NimPrinter().printConstRef(message, val)
+
+