[clang] [C++20][Modules] Fix false compilation error with constexpr (PR #143168)

[Dmitry Polukhin via cfe-commits]

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@@ -6781,7 +6781,7 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
       // and make sure we've initialized every step along it.
       auto IndirectFieldChain = IFD->chain();
       for (auto *C : IndirectFieldChain) {
-        FD = cast<FieldDecl>(C);
+        FD = cast<FieldDecl>(C)->getCanonicalDecl();
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dmpolukhin wrote:

> Instead of this change, this should just do `!declaresSameEntity(Value->getUnionField(), FD)` instead of `Value->getUnionField() != FD` a few lines below (outside of GitHub context).

Done, it is basically the same getCanonicalDecl but better for explaining the intent.

https://github.com/llvm/llvm-project/pull/143168