recog: Restore builds with Clang

Using parameter packs with function typedefs tripped a Clang bug
in which the packs were not being expanded correctly:

  https://bugs.llvm.org/show_bug.cgi?id=46377

Work around that by going back to the decltype approach, but adding
a cast to void to suppress a warning about unused values.

2020-06-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* coretypes.h (first_type): Delete.
	* recog.h (insn_gen_fn::operator()): Go back to using a decltype.

gcc/coretypes.h

@@ -359,10 +359,6 @@ struct kv_pair
   const ValueType value;	/* the value of the name */
 };
 
-/* Alias of the first type, ignoring the second.  */
-template<typename T1, typename T2>
-using first_type = T1;
-
 /* Iterator pair used for a collection iteration with range-based loops.  */
 
 template<typename T>

gcc/recog.h

@@ -297,7 +297,7 @@ struct insn_gen_fn
   template<typename ...Ts>
   rtx_insn *operator() (Ts... args) const
   {
-    typedef rtx_insn *(*funcptr) (first_type<rtx, Ts>...);
+    typedef rtx_insn *(*funcptr) (decltype ((void) args, NULL_RTX)...);
     return ((funcptr) func) (args...);
   }