Update stale comment in AsmWriter.cpp

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.

Patch by Alex Yursha.

Reviewers: gribozavr, vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45895

llvm-svn: 330962

llvm/lib/IR/AsmWriter.cpp

@@ -7,7 +7,9 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// 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.