[clang][deps] Skip writing `DIAG_PRAGMA_MAPPINGS` record (#70874)

Following up on #69975, this patch skips writing `DIAG_PRAGMA_MAPPINGS`
as well. Deserialization of this PCM record is still showing up in
profiles, since it needs to be VBR-decoded for every transitively loaded
PCM file.

The scanner doesn't make any guarantees about diagnostic accuracy (and
it even disables all warnings), so skipping this record should be safe.