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.

llvm-svn: 240529
diff --git a/llvm/lib/Object/Object.cpp b/llvm/lib/Object/Object.cpp
index 85f2436..42da2e4 100644
--- a/llvm/lib/Object/Object.cpp
+++ b/llvm/lib/Object/Object.cpp
@@ -187,7 +187,7 @@
 }
 
 uint64_t LLVMGetSymbolSize(LLVMSymbolIteratorRef SI) {
-  return (*unwrap(SI))->getSize();
+  return (*unwrap(SI))->getCommonSize();
 }
 
 // RelocationRef accessors