AArch6/ARMTargetParser.h - move Triple.h dependency down to cpp file. NFC.

Reduce Triple.h include to a forward declaration in the header. Only the implementations in the cpp files need the actual Triple class definition.
diff --git a/llvm/lib/Support/AArch64TargetParser.cpp b/llvm/lib/Support/AArch64TargetParser.cpp
index cce2d82..a6de446 100644
--- a/llvm/lib/Support/AArch64TargetParser.cpp
+++ b/llvm/lib/Support/AArch64TargetParser.cpp
@@ -12,8 +12,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/AArch64TargetParser.h"
-#include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/ADT/Triple.h"
 #include <cctype>
 
 using namespace llvm;