FusedLoc::get(context, locs) may return UnknownLoc when locs is empty
and no metadata is provided. The bytecode reader's cBuilder used
cast<FusedLoc>() on this result, which crashes with an assertion
failure.
Fix by giving the FusedLoc DialectAttribute its own cBuilder that passes
Attribute() explicitly, causing getChecked<FusedLoc> to call the
two-parameter storage constructor directly and always produce a
FusedLoc.
Fixes#99626
Assisted-by: Claude Code
aa4c76d8fb