[clang] [Sema] Add -Wc++11-narrowing-const-reference (PR #76094)

[Fangrui Song via cfe-commits]

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@@ -6158,12 +6158,24 @@ def err_illegal_initializer_type : Error<"illegal initializer type %0">;
 def ext_init_list_type_narrowing : ExtWarn<
   "type %0 cannot be narrowed to %1 in initializer list">,
   InGroup<CXX11Narrowing>, DefaultError, SFINAEFailure;
+// *_narrowing_const_reference diagnostics have the same messages, but are
+// controlled by -Wc++11-narrowing-const-reference for narrowing involving a
+// const reference.
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MaskRay wrote:

I created #77278 to simplify the code.

https://github.com/llvm/llvm-project/pull/76094