Use llvm-config.h instead of config.h

To fix standalone builds broken by r318411 (config.h is private to llvm).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@318514 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Frontend/PrecompiledPreamble.cpp

@@ -24,7 +24,7 @@
 #include "clang/Serialization/ASTWriter.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
-#include "llvm/Config/config.h"
+#include "llvm/Config/llvm-config.h"
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Mutex.h"