[dcl.attr.annotation] Disallow annotations on "remote" declarations

[katzdm]

Full name of submitter: Dan Katz

Reference: [dcl.attr.annotation]

Issue description
Other than new and novel forms of friend injection, there is currently very little motivation for allowing annotations on declarations of block-scope function and extern variable declarations, or on friend declarations. Consider:

template <int>
class Counter {
  struct S;
public:
  template <auto = []{}>
  static consteval size_t value() {
    void f(S);

    constexpr size_t result = annotations_of(^^f).size();
    return result;
  }

  template <auto = []{}>
  static consteval size_t incr() {
    [[=1]] void f(S);

    constexpr size_t result = annotations_of(^^f).size();
    return result;
  }
};

static_assert(Counter<0>::value() == 0);
static_assert(Counter<0>::incr() == 1);
static_assert(Counter<0>::incr() == 2);
static_assert(Counter<1>::incr() == 1);
static_assert(Counter<0>::incr() == 3);
static_assert(Counter<0>::value() == 3);
static_assert(Counter<1>::value() == 1);

(note: the above is currently IFNDR, but will be well-formed if the solution proposed for #808 is accepted; these issues are therefore related)

We should just do the conservative thing and disallow such annotations for C++26; ~nobody will miss them.

Suggested resolution
Modify [dcl.attr.annotation]/1 as follows:

An annotation may be applied to any a declaration of a type, type alias, variable, function, namespace, enumerator, base-specifier, or non-static data member. No annotation shall apply to a declaration D, nor to a function parameter declaration in the function-declarator (if any) of D, if either

• D is a friend declaration ([class.friend]) or
• the host scope of D ([basic.scope.scope]) is not the target scope of D.

[daveedvdv]

That last bit is a tad too strict. Annotations should be permitted on friend definitions with no other matching declaration in the TU.