[TableGen] Remove unnecessary include and just forward declare. NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238179 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/TableGen/TableGenBackend.h

@@ -14,10 +14,9 @@
 #ifndef LLVM_TABLEGEN_TABLEGENBACKEND_H
 #define LLVM_TABLEGEN_TABLEGENBACKEND_H
 
-#include "llvm/ADT/StringRef.h"
-
 namespace llvm {
 
+class StringRef;
 class raw_ostream;
 
 /// emitSourceFileHeader - Output an LLVM style file header to the specified