Combine multiple suppressions applied to the same diagnostic

[rjmholt]

PR Summary

Fixes #1691.

Instead of emitting a suppressed record for each suppression on a diagnostic, we combine all suppressions that apply to a diagnostic into a single object.

This partially reimplements #1694.

/cc @t-lipingma

PR Checklist

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[rjmholt]

@JamesWTruher elaborated on error reasoning here

[rjmholt]

string may change to String

Our preferred style is to use the language aliases over the full type names. For example string instead of String and int instead of Int32.

[bergmeister]

Looks good from a high level. I agree this PR is probably preferential to the original one.

[rjmholt]

Note that suppression attributes didn't seem to work in PSv4 until I made the change in 498a8c9