[flang] Changed ```PushConstruct()``` to be a template, which moves the implicit

creation of the ```ConstructNode``` into ```PushConstruct()```.

Original-commit: flang-compiler/f18@3984566858743c1b18a7a53cb899e68bf951154a
Reviewed-on: https://github.com/flang-compiler/f18/pull/686
Tree-same-pre-rewrite: false