[clang] [Clang][Sema] Error message refers to `__begin1` with `constexpr` variable in range-based for loop (PR #218145)

[Mariya Podchishchaeva via cfe-commits]

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@@ -2961,6 +2961,10 @@ def err_for_range_decl_must_be_var : Error<
 def err_for_range_storage_class : Error<
   "%select{loop|expansion}0 variable %1 may not be declared %select{'extern'|'static'|"
   "'__private_extern__'|'auto'|'register'|'constexpr'|'thread_local'}2">;
+def err_for_range_constexpr_loop_var : Error<
+  "constexpr variable %0 must be initialized by a constant expression; "
+  "a range-based for loop variable is initialized on each iteration from the "
+  "loop's iterator, whose value is not known at compile time">;
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Fznamznon wrote:

If we go that way IMO that should be a note in addition to existing `err_constexpr_var_requires_const_init` instead fully copy-paste of an existing message with additional test.

https://github.com/llvm/llvm-project/pull/218145