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