DebugInfo: Provide a utility for building a mapping from llvm::Function*s to llvm::DISubprograms

Update DeadArgumentElimintation to use this, with the intent of reusing
the functionality for ArgumentPromotion as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212122 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfo.h

@@ -934,6 +934,9 @@ private:
   /// Specify if TypeIdentifierMap is initialized.
   bool TypeMapInitialized;
 };
+
+DenseMap<Function *, DISubprogram> makeSubprogramMap(Module &M);
+
 } // end namespace llvm
 
 #endif