Move a method from IdentifierTable.h out of line and remove the SmallString include.

Fix all the transitive include users.

llvm-svn: 149783
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index 623ee1ff..24da01a 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -16,6 +16,7 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/StmtVisitor.h"
+#include "llvm/ADT/SmallString.h"
 using namespace clang;
 using namespace clang::serialization;