fixes "double friend" issue

[cjdb]

C++ specifiers can go in almost any order, but the friend logic was overlooking this for some cases. In particular, constexpr friend wasn't being accounted for.

The following cases have been verified.

struct cat {
  /// Checks if the cats are the same.
  constexpr friend bool operator==(cat x, cat y)
  {
    return true;
  }

  /// Makes the cat cute.
  friend void make_cute(cat c)
  {
  }

  /// Makes the cat cuter.
  friend void make_cuter(cat c);

  /// Makes the cat the cutest.
  constexpr friend void make_cutest(cat c);
};

/// This won't be picked up.
constexpr void make_cutest(cat c)
{}

Fixes #916

[cjdb]

Is there a way to add unit tests? I haven't worked out how to do that for this project yet :(

[emilydolson]

Since I have a vested interest in this getting merged (in that I would love for it to be fixed, and also for the fix for #917 to get merged), I messed around with the testing infrastructure and I wrote what I believe is a sufficient test for this fix. @cjdb, I'm sending it to you as a PR on the friend branch of your fork.