[clang] [clang][Sema] fix incorrect ambiguous function call which use designated-initializer as template argument (PR #167159)

[Erich Keane via cfe-commits]

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@@ -5628,6 +5628,11 @@ class DesignatedInitExpr final
       FieldInfo.NameOrField = reinterpret_cast<uintptr_t>(FD);
     }
 
+    void clearFieldDecl() {
+      assert(isFieldDesignator() && "Only valid on a field designator");
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erichkeane wrote:

Why is this necessary?  The fact that we need this is probably meaning we're doing something VERY wrong.

https://github.com/llvm/llvm-project/pull/167159