Don't allocate srclocs if we only have default SourceLoc values.
Don't allocate srclocs if we only have default SourceLoc values.

This avoids needless extra indentation in `write_function` in the
case where no SourceLoc information is available.

sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne