Revert "[Driver] Use llvm-config.h, not config.h to unbreak out-of-tree builds"

This reverts commit r279035. According to Richard Smith, llvm-config.h
does not contain the right definitions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279097 91177308-0d34-0410-b5e6-96231b3b80d8

tools/driver/cc1_main.cpp

@@ -25,7 +25,7 @@
 #include "clang/Frontend/Utils.h"
 #include "clang/FrontendTool/Utils.h"
 #include "llvm/ADT/Statistic.h"
-#include "llvm/Config/llvm-config.h"
+#include "llvm/Config/config.h"
 #include "llvm/LinkAllPasses.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/OptTable.h"