[lldb] Fix progress reporting for SymbolLocatorDebugSymbols (#79624)

This fixes two issues related to the DebugSymbols symbol locator:

1. Only the default symbol locator plugin reports progress. On Darwin,
which uses the DebugSymbols framework we need to report the same
progress form the corresponding SymbolLocator plugin.

2. Forceful dSYM lookups, for example when using `add-dsym`, use a
different code path that currently does not report progress, which is
confusing. Here the progress event can be more specific and specify its
downloading a symbol file rather than just locating it as we'll always
shell out to dsymForUUID or its equivalent.

rdar://121629777