[flang] Add passArg to ProcEntityDetails and ProcBindingDetails

Add `passArg` to `ProcEntityDetails` and `ProcBindingDetails`.
The is the symbols that represents the passed-object dummy argument,
or nullptr if none.

As both of these classes need `passName` and `passArg`, create a mixin
class, `WithPassArg` for the shared content.

Change passName_ from optional to pointer because that makes it a little
easier to work with and there is no need to have a copy of the name.

Original-commit: flang-compiler/f18@1cdd1a5b87016c5aff0fdc571532c25253b91938
Reviewed-on: https://github.com/flang-compiler/f18/pull/521
Tree-same-pre-rewrite: false