Move TableGen's parser and entry point into a library

This is the first step towards splitting LLVM and Clang's tblgen executables.

llvm-svn: 140951
diff --git a/llvm/utils/TableGen/SubtargetEmitter.h b/llvm/utils/TableGen/SubtargetEmitter.h
index b239f3d..ff01274 100644
--- a/llvm/utils/TableGen/SubtargetEmitter.h
+++ b/llvm/utils/TableGen/SubtargetEmitter.h
@@ -14,7 +14,7 @@
 #ifndef SUBTARGET_EMITTER_H
 #define SUBTARGET_EMITTER_H
 
-#include "TableGenBackend.h"
+#include "llvm/TableGen/TableGenBackend.h"
 #include "llvm/MC/MCInstrItineraries.h"
 #include <vector>
 #include <map>