[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Chuanqi Xu via cfe-commits]

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@@ -12854,6 +12854,15 @@ def err_invalid_module_name : Error<"%0 is an invalid name for a module">;
 def err_extern_def_in_header_unit : Error<
   "non-inline external definitions are not permitted in C++ header units">;
 
+def err_meaningless_export : Error<
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ChuanqiXu9 wrote:

ISO spec asks us to emit diagnostics and we've already done that. It is fine. And specifically, it is actually not hurting to export them. Making them a hard error may only breaking user's code.

https://github.com/llvm/llvm-project/pull/194201