Change how symbol sizes are handled in lib/Object.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240529 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/MCTargetDesc/X86ELFRelocationInfo.cpp

@@ -31,8 +31,8 @@ public:
 
     StringRef SymName; SymI->getName(SymName);
     uint64_t  SymAddr; SymI->getAddress(SymAddr);
-    uint64_t SymSize = SymI->getSize();
     auto *Obj = cast<ELFObjectFileBase>(Rel.getObjectFile());
+    uint64_t SymSize = Obj->getSymbolSize(*SymI);
     int64_t Addend = *Obj->getRelocationAddend(Rel.getRawDataRefImpl());
 
     MCSymbol *Sym = Ctx.getOrCreateSymbol(SymName);