Create a stub mapping for truncated nodes

[kolesnikovae]

When a pprof profile is truncated, we replace the removed nodes with a shared "stub" location (named "other") that references the default mapping (ID == 1), which may lead to ambiguity. Instead, a shared "stub" mapping should be created (probably, even if the profile already has a "stub" mapping created at ingestion).

[simonswine]

Yes I do agree, esp. once we want to attach more information (OTEL spec) to the mapping we should be careful what belongs together or not.

[kolesnikovae]

@marc It might be crucial for read-path symbolication (e.g., #4203)