[flang] Replace scope stack with current scope

[flang] Replace scope stack with current scope

We don't need to manage a stack of scopes: each scope has a reference to
its parent, so popping the scope stack is equivalent to setting the
new current scope to the parent of the old current scope.

Original-commit: flang-compiler/f18@022aa7a7f9d09ee96db6ec9a025afb6bcf99cc55
Reviewed-on: https://github.com/flang-compiler/f18/pull/167
Tree-same-pre-rewrite: false