c++: Don't add built-in operator for ++ on bool.

c++: Don't add built-in operator for ++ on bool.

This feels extremely obscure but at least it's an opportunity to fix the
comments.  P0002R1 removed deprecated operator++(bool) in C++17 so let's
avoid adding a builtin overload candidate for ++ when the type is bool.

	* call.c (add_builtin_candidate): Don't create a builtin overload
	candidate for ++ when type is bool in C++17.

	* g++.dg/overload/builtin5.C: New test.