recog: Restore builds with Clang

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.