[Clang][Sema] Diagnose friend declarations with enum elaborated-type-specifier...

[Clang][Sema] Diagnose friend declarations with enum elaborated-type-specifier in all language modes (#80171)

According to [dcl.type.elab] p4:
> If an _elaborated-type-specifier_ appears with the `friend` specifier
as an entire _member-declaration_, the _member-declaration_ shall have
one of the following forms:
>     `friend` _class-key_ _nested-name-specifier_(opt) _identifier_ `;`
>     `friend` _class-key_ _simple-template-id_ `;`
> `friend` _class-key_ _nested-name-specifier_ `template`(opt)
_simple-template-id_ `;`

Notably absent from this list is the `enum` form of an
_elaborated-type-specifier_ "`enum` _nested-name-specifier_(opt)
_identifier_", which appears to be intentional per the resolution of
CWG2363.

Most major implementations accept these declarations, so the diagnostic
is a pedantic warning across all C++ versions.

In addition to the trivial cases previously diagnosed in C++98, we now
diagnose cases where the _elaborated-type-specifier_ has a dependent
_nested-name-specifier_:
```
template<typename T>
struct A
{
    enum class E;
};

struct B
{
    template<typename T>
    friend enum A<T>::E; // pedantic warning: elaborated enumeration type cannot be a friend
};

template<typename T>
struct C
{
    friend enum T::E;  // pedantic warning: elaborated enumeration type cannot be a friend
};
```