llvm-readobj: handle import libraries with -coff-exports

`link -dump -exports` lists exported symbols from import libraries as well as
normal dlls.  Ensure that we can handle import libraries as well in
llvm-readobj.

llvm-svn: 279069
diff --git a/llvm/tools/llvm-readobj/llvm-readobj.cpp b/llvm/tools/llvm-readobj/llvm-readobj.cpp
index 14178b9..d811ea9 100644
--- a/llvm/tools/llvm-readobj/llvm-readobj.cpp
+++ b/llvm/tools/llvm-readobj/llvm-readobj.cpp
@@ -460,6 +460,8 @@
     }
     if (ObjectFile *Obj = dyn_cast<ObjectFile>(&*ChildOrErr.get()))
       dumpObject(Obj);
+    else if (COFFImportFile *Imp = dyn_cast<COFFImportFile>(&*ChildOrErr.get()))
+      dumpCOFFImportFile(Imp);
     else
       reportError(Arc->getFileName(), readobj_error::unrecognized_file_format);
   }