[NFC] Header cleanup

Summary: Removed unused headers, replaced some headers with forward class declarations

Patch by: Eugene <claprix@yandex.ru>

Differential Revision: https://reviews.llvm.org/D20100

llvm-svn: 275882
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index bb6a8ae..314eee0 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -12,7 +12,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Serialization/ASTWriter.h"
-#include "clang/Serialization/ModuleFileExtension.h"
 #include "ASTCommon.h"
 #include "ASTReaderInternals.h"
 #include "MultiOnDiskHashTable.h"
@@ -44,6 +43,7 @@
 #include "clang/Sema/IdentifierResolver.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Serialization/ASTReader.h"
+#include "clang/Serialization/ModuleFileExtension.h"
 #include "clang/Serialization/SerializationDiagnostic.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
@@ -52,7 +52,6 @@
 #include "llvm/Bitcode/BitstreamWriter.h"
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/EndianStream.h"
-#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/OnDiskHashTable.h"
 #include "llvm/Support/Path.h"