Style notes on passing and storing object addresses

[jonmeow]

Context. Some relevant Google C++ style is at Inputs and Outputs. #4301 is an example application of the style.

[zygoloid]

LGTM

[chandlerc]

Based on further discussion, let's keep the storing part but add some more explicit guidance for passing. This is allowed and slightly encouraged in the Google style guide, but I think we can be a bit more direct and explicit for Carbon:

Suggested change

	- When storing unowned objects as members, prefer to pass by reference and
	store as a pointer. For example, `Foo* foo_;` and
	`Bar(Foo& foo) : foo_(&foo)`.
	- When passing a reference to an unowned object as an argument, use a reference
	unless one of the following cases applies:
	- If it is optional, use a pointer and document that it may be null.
	- If it is captured and must outlive the call expression itself, use a
	pointer an document that it must not be null (unless it is also optional).
	- When storing a reference to an unowned object as a member, prefer to store as a
	pointer. For example, `Foo* foo_;` and `Bar(Foo* foo) : foo_(foo)`.

[jonmeow]

Pulled, and made some edits (including "when passing/storing a reference to an unowned object" -> "when passing/storing an object's address" in order to avoid different meanings of "reference", cleaning up the example)

[chandlerc]

I think the thread is coming to consensus now, and suggested an update to match what I think I'm seeing there... but also maybe I'm misreading so good to use the suggested change to make sure we're all on the same page.

[chandlerc]

LGTM, nice rewording too. =]