fixes "double friend" issue

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.

```cpp
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 breathe-doc#916

breathe/renderer/sphinxrenderer.py

@@ -1975,27 +1975,26 @@ def visit_function(self, node) -> List[Node]:
                 )
             else:
                 elements = [self.create_template_prefix(node)]
+                # TODO: handle constexpr when parser has been updated
+                #       but Doxygen seems to leave it in the type anyway
+                typ = "".join(n.astext() for n in self.render(node.get_type()))
+                # Doxygen sometimes leaves 'static' in the type,
+                # e.g., for "constexpr static auto f()"
+                typ = typ.replace("static ", "")
                 if node.static == "yes":
                     elements.append("static")
                 if node.inline == "yes":
                     elements.append("inline")
                 if node.kind == "friend":
+                    # In Doxygen up to somewhere between 1.8.17 to exclusive 1.9.1
+                    # the 'friend' part is also left in the type.
+                    # See also: #767, #916
+                    typ = re.sub(r"(^|\s+)friend(\s+)", r"\1\2", typ)
                     elements.append("friend")
                 if node.virt in ("virtual", "pure-virtual"):
                     elements.append("virtual")
                 if node.explicit == "yes":
                     elements.append("explicit")
-                # TODO: handle constexpr when parser has been updated
-                #       but Doxygen seems to leave it in the type anyway
-                typ = "".join(n.astext() for n in self.render(node.get_type()))
-                # Doxygen sometimes leaves 'static' in the type,
-                # e.g., for "constexpr static auto f()"
-                typ = typ.replace("static ", "")
-                # In Doxygen up to somewhere between 1.8.17 to exclusive 1.9.1
-                # the 'friend' part is also left in the type.
-                # See also #767.
-                if typ.startswith("friend "):
-                    typ = typ[7:]
                 elements.append(typ)
                 elements.append(name)
                 elements.append(node.get_argsstring())