[-cxx-abi microsoft] Emit linkonce_odr definitions for declarations of static data members with inline initializers (PR17689)

This makes Clang emit a linkonce_odr definition for 'val' in the code below,
to be compatible with MSVC-compiled code:

  struct Foo {
    static const int val = 1;
  };

Differential Revision: http://llvm-reviews.chandlerc.com/D2233

llvm-svn: 195283
diff --git a/clang/lib/CodeGen/CGCXXABI.h b/clang/lib/CodeGen/CGCXXABI.h
index 44bdf66..9e9a2a7 100644
--- a/clang/lib/CodeGen/CGCXXABI.h
+++ b/clang/lib/CodeGen/CGCXXABI.h
@@ -373,6 +373,10 @@
   /// Gets the deleted virtual member call name.
   virtual StringRef GetDeletedVirtualCallName() = 0;
 
+  /// \brief Returns true iff static data members that are initialized in the
+  /// class definition should have linkonce linkage.
+  virtual bool isInlineInitializedStaticDataMemberLinkOnce() { return false; }
+
   /**************************** Array cookies ******************************/
 
   /// Returns the extra size required in order to store the array