[clang] [clang][NFC] Add test for CWG issues about linkage in cross-TU context (PR #113736)

[Vlad Serebrennikov via cfe-commits]

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@@ -0,0 +1,548 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file --leading-lines %s %t
+// RUN: %clang_cc1 -std=c++20 -pedantic-errors -fexceptions -fcxx-exceptions %t/cwg1884_A.cppm -triple x86_64-unknown-unknown -emit-module-interface -o %t/cwg1884_A.pcm
+// RUN: %clang_cc1 -std=c++20 -verify=since-cxx20 -pedantic-errors -fexceptions -fcxx-exceptions -triple x86_64-unknown-unknown %t/cwg1884.cpp -fmodule-file=cwg1884_A=%t/cwg1884_A.pcm
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Endilll wrote:

I don't want readers of the test to have a misconception that this is tested in C++17 and earlier modes. Even you just paid attention to the fact that this DR, which is applied to C++98, is only tested in C++20 mode and newer. So I consider using `since-cxx20` prefix a feature.

https://github.com/llvm/llvm-project/pull/113736