[clang] [RFC][C++20][Modules] Relax ODR check in unnamed modules (PR #111160)

[Dmitry Polukhin via cfe-commits]

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@@ -2527,7 +2527,7 @@ class BitsUnpacker {
 
 inline bool shouldSkipCheckingODR(const Decl *D) {
   return D->getASTContext().getLangOpts().SkipODRCheckInGMF &&
-         D->isFromGlobalModule();
+         (D->isFromGlobalModule() || !D->isInNamedModule());
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dmpolukhin wrote:

Added `isFromHeaderUnits()` and use it now.

But in general from my reading of C++ standard header units should treated as GMF see [10.3p5 note 2](https://eel.is/c++draft/module#import-note-2):
> [Note 2: A header unit is a separate translation unit with an independent set of defined macros. All declarations within a header unit are implicitly exported ([module.interface]), and are attached to the global module ([module.unit]). — end note]

So perhaps `isFromGlobalModule` should also include header units, it is used only in very few places. WDYT?

https://github.com/llvm/llvm-project/pull/111160