Add a non-templated ELFObjectFileBase class.

Use it to implement some ELF only virtual interfaces instead of using error
prone series of dyn_casts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215838 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Object/ELFObjectFile.cpp

@@ -17,6 +17,10 @@
 namespace llvm {
 using namespace object;
 
+ELFObjectFileBase::ELFObjectFileBase(unsigned int Type,
+                                     std::unique_ptr<MemoryBuffer> Source)
+    : ObjectFile(Type, std::move(Source)) {}
+
 ErrorOr<std::unique_ptr<ObjectFile>>
 ObjectFile::createELFObjectFile(std::unique_ptr<MemoryBuffer> &Obj) {
   std::pair<unsigned char, unsigned char> Ident =