Fix bug 19437 - Only add discriminators for DWARF 4 and above.

Summary:
This prevents the discriminator generation pass from triggering if
the DWARF version being used in the module is prior to 4.

Reviewers: echristo, dblaikie

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3413

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206507 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Module.h

@@ -598,6 +598,14 @@ public:
   /// is delete'd for real.  Note that no operations are valid on an object
   /// that has "dropped all references", except operator delete.
   void dropAllReferences();
+
+/// @}
+/// @name Utility functions for querying Debug information.
+/// @{
+
+  /// \brief Returns the Dwarf Version by checking module flags.
+  unsigned getDwarfVersion() const;
+
 /// @}
 };