Update stale comment in AsmWriter.cpp

[yursha]

Summary

The old comment referred to llvm/IR/Writer.h which doesn't longer exist.
This patch replaces it with an up-to-date description of AsmWriter library.

lib/IR/AsmWriter.cpp

 //===- AsmWriter.cpp - Printing LLVM as an assembly file ------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
-// This library implements the functionality defined in llvm/IR/Writer.h
+// This library implements `print` family of functions in classes like
+// Module, Function, Value, etc. In-memory representation of those classes is
+// converted to IR strings.
 //
 // Note that these routines must be extremely tolerant of various errors in the
 // LLVM code, because it can be used for debugging transformations.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"