Move comment to correct function.
Extend SpecificImageDebugInfo::GetFunctions() ...

...to also take the list of symbols in the image as a parameter.
This allows us to prefetch the symbols once in
ImageInfo::FinishInit() for each image and then let each specific info
subclass use them if/as needed, rather than having to do the entire
symbol lookup/sort twice for every image.

- Adjust callers accordingly.
DwarfFile: Add hook to query availability of frame unwind info.
DwarfImageDebugInfo: Minor optimization.

In the fallback case where full debug information is unavailable,
query the underlying DWARF file to see if CFI is available at all.
If it isn't, skip creating function debug info objects.