Split alignment calculation from layout calculation

[oskgo]

This is a refactoring to simplify future work on alignment such as in #11924 and rust-lang/rfcs#3204.

[rustbot]

r? @eholk

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[oskgo]

r? ghost

[rustbot]

Failed to set assignee to ghost: invalid assignee

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[eholk]

@rustbot author

(Setting as waiting on author since this is marked as a draft still)

[oskgo]

I don't have the confidence to make these changes without mentoring yet, so I'll close this. I might pick it back up once I'm more comfortable or if someone wants to mentor work on alignment, but this PR can be considered abandoned.