Reuse inst fingerprinter across multiple manglings

Description of the bug:

For each symbol we lower, we create a new SemIR::Mangler, which creates a new InstFingeprinter, which means we don't cache any fingerprint computations across manglings. This could in principle add up to a lot of repeated work. We should cache a single SemIR::Mangler in the FileContext to share a fingerprint cache.

What did you do, or what's a simple way to reproduce the bug?

No response

What did you expect to happen?

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What actually happened?

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Any other information, logs, or outputs that you want to share?

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