explicit conversion functions are incompatible with C++98 not warning in C++20

[shafik]

Given:

struct HasExplicitConversion {
  explicit operator bool(); 
};

Using -Wc++98-compat in C+11/14/17 we obtain the following diagnostic:

warning: explicit conversion functions are incompatible with C++98 [-Wc++98-compat]
    2 |   explicit operator bool(); 
      |   ^~~~~~~~

but not in C++20. This change came in w/ 76b9027

[shafik]

CC @zygoloid