On ELF that was already the case since getting the size of a symbol
never fails.
On MachO and COFF we could fail trying to get the section of a symbol. But
we don't really need the section, just the section number to know if two
symbols are in the same section or not.
llvm-svn: 240580
COFF and MachO only define symbol sizes for common symbols. Reflect that
in the class hierarchy by having a method for common symbols only in the base
and a general one in ELF.
This avoids the need of using a magic value for the size, which had a few
problems
* Most callers didn't check for it.
* The ones that did could not tell the magic value from a file actually having
that value.
llvm-svn: 240529
