llvm 14: handle move of TargetRegister.h
For llvm 14 TargetRegister.h moved from llvm/Support to
llvm/MC.  So adjust how it is included.
IGNORE_COMPILER_WARNING("-Wunused-parameter")
#include <llvm/Support/Casting.h>
+#ifdef LLVM_OLDER_THAN_14_0
#include <llvm/Support/TargetRegistry.h>
+#else
+#include <llvm/MC/TargetRegistry.h>
+#endif
#include <llvm/Support/SourceMgr.h>
#include <llvm/Support/CommandLine.h>
