[clang] [Clang][NFCI] Cleanup the fix for default function argument substitution (PR #104911)

[Younan Zhang via cfe-commits]

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@@ -285,7 +286,8 @@ Response HandleFunction(Sema &SemaRef, const FunctionDecl *Function,
     // If this function was instantiated from a specialized member that is
     // a function template, we're done.
     assert(Function->getPrimaryTemplate() && "No function template?");
-    if (Function->getPrimaryTemplate()->isMemberSpecialization())
+    if (!ForDefaultArgumentSubstitution &&
+        Function->getPrimaryTemplate()->isMemberSpecialization())
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zyn0217 wrote:

An alternative is to use `(ForConstraintInstantiation || !RelativeToPrimary)` so we could get rid of the usages for concepts where it expects an end after a member specialization. But I feel like that makes the semantics of the flags vague, so ideas are welcome here.

https://github.com/llvm/llvm-project/pull/104911