Rollup merge of #60467 - nnethercote:less-symbol-interning, r=davidtwco

Avoid repeated interning of static strings.

`file_metadata_raw` interns the strings `"<unknown>"` and `""` very
frequently. This commit avoids that, which reduces the number of symbols
interned significantly and reduces instruction counts by up to 0.5% on
some workloads.