[lldb][SymbolFileDWARFDebugMap] Introduce enum to indicate whether to continue iteration of object files (#87344)

This patch introduces a new `IterationMarker` enum (happy to take
alternative name suggestions), which callbacks, like the one in
`SymbolFileDWARFDebugMap::ForEachSymbolFile`, can return in order to
indicate whether the caller should continue iterating or bail.

For now this patch just changes the `ForEachSymbolFile` callback to use
this new enum. In the future we could change the various
`DWARFIndex::GetXXX` callbacks to do the same.

This makes the callbacks easier to read and hopefully reduces the chance
of bugs like https://github.com/llvm/llvm-project/pull/87177.

GitOrigin-RevId: e61d6b74ddf28df196484f6251271f543ae902ab
3 files changed