[Object][Archive][NFC] Create all symbolic files objects before calculating offsets. (#85229)

This is refactoring preparing to move UseECMap computation to the
archive writer. We currently require writeArchive caller to pass that.
This is not practical for llvm-ar, which currently interprets at most
one passed object. For a reliable UseECMap, we need to interpret all
symbolic objects: we may have, for example, a list of x86_64 files
followed by aarch64 file, which indicates that we should use EC map for
x86_64 objects.

This commit interprets symbolic files in a separated pass, which will be
a convenient place to implement UseECMap computation in the follow up.
It also makes accessing the next member for AIX big archive offset
computation a bit easier.