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

Author: MitalAshok

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};

clang-tools-extra/clangd/unittests/HoverTests.cpp

@@ -2284,7 +2284,7 @@ TEST(Hover, All) {
             namespace std
             {
               template<class _E>
-              class initializer_list {};
+              class initializer_list { const _E *a, *b; };
             }
             void foo() {
               ^[[auto]] i = {1,2};

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

@@ -945,7 +945,7 @@ TEST(ParameterHints, ConstructorStdInitList) {
   // Do not show hints for std::initializer_list constructors.
   assertParameterHints(R"cpp(
     namespace std {
-      template <typename> class initializer_list {};
+      template <typename E> class initializer_list { const E *a, *b; };
     }
     struct S {
       S(std::initializer_list<int> param);

clang-tools-extra/clangd/unittests/XRefsTests.cpp

@@ -771,7 +771,7 @@ TEST(LocateSymbol, All) {
         namespace std
         {
           template<class _E>
-          class [[initializer_list]] {};
+          class [[initializer_list]] { const _E *a, *b; };
         }
 
         ^auto i = {1,2};

clang-tools-extra/include-cleaner/unittests/WalkASTTest.cpp

@@ -534,7 +534,7 @@ TEST(WalkAST, Enums) {
 TEST(WalkAST, InitializerList) {
   testWalk(R"cpp(
        namespace std {
-        template <typename T> struct $implicit^initializer_list {};
+        template <typename T> struct $implicit^initializer_list { const T *a, *b; };
        })cpp",
            R"cpp(
        const char* s = "";

clang-tools-extra/test/clang-tidy/checkers/modernize/min-max-use-initializer-list.cpp

@@ -11,6 +11,7 @@ T max(T a, T b) {
 namespace std {
 template< class T >
 struct initializer_list {
+  const T *a, *b;
   initializer_list()=default;
   initializer_list(T*,int){}
   const T* begin() const {return nullptr;}

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 {}
 };
 

clang-tools-extra/test/clang-tidy/checkers/modernize/use-emplace.cpp

@@ -8,9 +8,10 @@
 // RUN:                '::std::make_pair; ::std::make_tuple; ::test::MakeSingle'}}"
 
 namespace std {
-template <typename>
+template <typename E>
 class initializer_list {
 public:
+  const E *a, *b;
   initializer_list() noexcept {}
 };
 

clang-tools-extra/test/clang-tidy/checkers/performance/inefficient-vector-operation.cpp

@@ -5,7 +5,7 @@
 
 namespace std {
 
-typedef int size_t;
+typedef decltype(sizeof 0) size_t;
 
 template<class E> class initializer_list {
 public:
@@ -15,6 +15,8 @@ template<class E> class initializer_list {
   using size_type = size_t;
   using iterator = const E*;
   using const_iterator = const E*;
+  iterator p;
+  size_t sz;
   initializer_list();
   size_t size() const; // number of elements
   const E* begin() const; // first element

clang-tools-extra/test/clang-tidy/checkers/readability/isolate-declaration-cxx17.cpp

@@ -31,7 +31,7 @@ struct SomeClass {
 
 namespace std {
 template <typename T>
-class initializer_list {};
+class initializer_list { const T *a, *b; };
 
 template <typename T>
 class vector {