Sink some PTHManager includes out of Preprocessor.h

This reduces the number of .cpp files needed to be rebuilt after
touching OnDiskHashTable from 120 to 21 for me.

llvm-svn: 251810
diff --git a/clang/lib/Frontend/CacheTokens.cpp b/clang/lib/Frontend/CacheTokens.cpp
index 7d2a09c..662778b 100644
--- a/clang/lib/Frontend/CacheTokens.cpp
+++ b/clang/lib/Frontend/CacheTokens.cpp
@@ -19,6 +19,7 @@
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Lex/Lexer.h"
+#include "clang/Lex/PTHManager.h"
 #include "clang/Lex/Preprocessor.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringMap.h"