[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, 2 insertions, 1 deletions

diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-emplace-ignore-implicit-constructors.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-emplace-ignore-implicit-constructors.cpp
index 2004993..150e3ac 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-emplace-ignore-implicit-constructors.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-emplace-ignore-implicit-constructors.cpp
@@ -4,10 +4,11 @@
 // RUN:                true}}"
 
 namespace std {
-template <typename>
+template <typename E>
 class initializer_list
 {
 public:
+  const E *a, *b;
   initializer_list() noexcept {}
 };