privacy: share effective visibility initialization

[SynapLink]

Address one FIXME in EffectiveVisibilities by sharing the private effective visibility initialization path between effective_vis_or_private and update.

update now mutates the map entry in place instead of copying the effective visibility out and inserting it back at the end. The inserted default value and update logic are unchanged.

Validation:

• git diff --check
• python x.py check compiler/rustc_middle

[rustbot]

r? @dingxiangfei2009

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