[clang] [clang-tools-extra] [Clang][Sema] Clarify structured binding diagnostic when initializer is missing (PR #181803)

[Oliver Hunt via cfe-commits]

================
@@ -606,7 +606,8 @@ def err_decomp_decl_template : Error<
 def err_decomp_decl_not_alone : Error<
   "structured binding declaration must be the only declaration in its group">;
 def err_decomp_decl_requires_init : Error<
-  "structured binding declaration %0 requires an initializer">;
+  "structured binding declaration %0 requires an initializer; "
+  "expected '=', '(', or a braced initializer list">;
----------------
ojhunt wrote:

The text in the error message isn't the issue - we don't need a change to this.

https://github.com/llvm/llvm-project/pull/181803