Properly wrap discarded statements (#10322)

Failing to do so lead the codegen to emit invalid code sometimes,
especially when C++ references were involved.

Fixes #10241

tests/cpp/t10241.nim

@@ -0,0 +1,19 @@
+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): var String
+    {.importcpp: "append", header: "string", discardable.}
+
+var
+  s1 = initString()
+  s2 = initString()
+
+s1.append s2