[Clang] [Sema] Diagnose unknown std::initializer_list layout in SemaInit (#95580)

This checks if the layout of `std::initializer_list` is something Clang
can handle much earlier and deduplicates the checks in
CodeGen/CGExprAgg.cpp and AST/ExprConstant.cpp

Also now diagnose `union initializer_list` (Fixes #95495), bit-field for
the size (Fixes a crash that would happen during codegen if it were
unnamed), base classes (that wouldn't be initialized) and polymorphic
classes (whose vtable pointer wouldn't be initialized).

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
index 2a74d7f..cfbc64c 100644
--- a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
+++ b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
@@ -3265,7 +3265,7 @@ TEST(TransferTest, ResultObjectLocationForStdInitializerListExpr) {
   std::string Code = R"(
     namespace std {
     template <typename T>
-    struct initializer_list {};
+    struct initializer_list { const T *a, *b; };
     } // namespace std
 
     void target() {