[Clang] Reword diagnostic for scope identifier with linkage

If the declaration of an identifier has block scope, and the identifier has
external or internal linkage, the declaration shall have no initializer for
the identifier.

Clang now gives a more suitable diagnosis for this case.
Fixes https://github.com/llvm/llvm-project/issues/57478



Signed-off-by: Jun Zhang <jun@junz.org>

 Differential Revision: https://reviews.llvm.org/D133088