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

[Victor Chernyakin via cfe-commits]

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@@ -3,9 +3,135 @@
 
 
 // RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/A.cpp
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/B.cpp
+// RUN: not %clang_cc1 -std=c++20 -fsyntax-only -fdiagnostics-parseable-fixits %t/B.cpp 2>&1 | FileCheck %t/B.cpp
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/msvc-stl-exception-1.cpp
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localspook wrote:

> I a bit concerns here: will this code snippet also take effect for non-MSVC-compatible code?

I guess if someone defines `_MSVC_STL_UPDATE` on another platform for some reason, it will take effect.

> We should avoid this cost even it's called only once.

Fair enough, added the check.

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