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

[Krystian Stasiowski via cfe-commits]

================
@@ -1637,10 +1637,8 @@ def err_inline_namespace_std : Error<
 def err_unexpected_friend : Error<
   "friends can only be classes or functions">;
 def ext_enum_friend : ExtWarn<
-  "befriending enumeration type %0 is a C++11 extension">, InGroup<CXX11>;
-def warn_cxx98_compat_enum_friend : Warning<
-  "befriending enumeration type %0 is incompatible with C++98">,
-  InGroup<CXX98Compat>, DefaultIgnore;
+  "elaborated enumeration type cannot be a friend">,
----------------
sdkrystian wrote:

> > ISO C++ does not permits this declaration to be declared as friend because 'enum E' is an elaborated enum specifier.
> 
> There. Something along these lines

Not a huge fan of this phrasing. The wording of the diagnostic should already imply that such a declaration is invalid. Moreover:
- "elaborated enum specifier" isn't any "more correct" than "elaborated enum type"
- "declaration to be declared" is a bit redundant
- the phrasing implies that the declaration might be well-formed when written without `friend` (which it isn't).

https://github.com/llvm/llvm-project/pull/80171