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/Frontend/HeaderIncludeGen.cpp b/clang/lib/Frontend/HeaderIncludeGen.cpp
index 3b41d89..f703674 100644
--- a/clang/lib/Frontend/HeaderIncludeGen.cpp
+++ b/clang/lib/Frontend/HeaderIncludeGen.cpp
@@ -11,6 +11,7 @@
 #include "clang/Basic/SourceManager.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Lex/Preprocessor.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace clang;