[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-tools-extra/clangd/unittests/ASTTests.cpp b/clang-tools-extra/clangd/unittests/ASTTests.cpp
index 3101bf3..32c8e8a 100644
--- a/clang-tools-extra/clangd/unittests/ASTTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ASTTests.cpp
@@ -80,7 +80,7 @@ TEST(GetDeducedType, KwAutoKwDecltypeExpansion) {
               namespace std
               {
                 template<class _E>
-                class [[initializer_list]] {};
+                class [[initializer_list]] { const _E *a, *b; };
               }
 
               ^auto i = {1,2};