[clang] [C++20][Modules] Fix incomplete decl chains (PR #129982)

[Michael Park via cfe-commits]

================
@@ -9180,6 +9180,12 @@ bool Sema::hasAcceptableDefinition(NamedDecl *D, NamedDecl **Suggested,
   if (!getLangOpts().Modules && !getLangOpts().ModulesLocalVisibility)
     return true;
 
+  // The external source may have additional definitions of this entity that are
+  // visible, so complete the redeclaration chain now.
+  if (auto *Source = Context.getExternalSource()) {
+    Source->CompleteRedeclChain(D);
+  }
----------------
mpark wrote:

Yes, calling `redecls` or `getMostRecentDecl` (similar to [CXXRecordDecl::dataPtr](https://github.com/llvm/llvm-project/blob/e4c3d258b7a1f335cfd3a90bcf3d28ea220c999d/clang/include/clang/AST/DeclCXX.h#L457-L461) here would work as well. The reason I used this "pattern" here is because of [this code](https://github.com/llvm/llvm-project/blob/d6dbfd6f01710b8fed2303e66f60903efd2283f2/clang/lib/Sema/SemaType.cpp#L9257-L9262) at the end of the same function.

I'm very much open to refining the solution. I wanted to submit this PR now because the unit-test-level repro was something we were missing from #126973 

https://github.com/llvm/llvm-project/pull/129982