Object/COFF: Add function to check if section number is reserved one.

Differential Revision: http://llvm-reviews.chandlerc.com/D3103

llvm-svn: 204199
diff --git a/llvm/tools/llvm-readobj/COFFDumper.cpp b/llvm/tools/llvm-readobj/COFFDumper.cpp
index e790e5c..f17524e 100644
--- a/llvm/tools/llvm-readobj/COFFDumper.cpp
+++ b/llvm/tools/llvm-readobj/COFFDumper.cpp
@@ -985,7 +985,7 @@
     } else if (
         Symbol->StorageClass   == COFF::IMAGE_SYM_CLASS_WEAK_EXTERNAL ||
         (Symbol->StorageClass  == COFF::IMAGE_SYM_CLASS_EXTERNAL &&
-         Symbol->SectionNumber == 0 &&
+         Symbol->SectionNumber == COFF::IMAGE_SYM_UNDEFINED &&
          Symbol->Value         == 0)) {
       const coff_aux_weak_external_definition *Aux;
       if (error(getSymbolAuxData(Obj, Symbol + I, Aux)))