Interop support for initialization via std::initializer_list. (#6672)

Add a new builtin function `cpp.std.initializer_list.make` that takes an
array and returns a `std::initializer_list`, initialized to refer to
that array. When C++ initialization wants to perform a
`std::initializer_list`-from-array construction, synthesize a
declaration of a matching builtin function and use that to perform the
initialization.

Ideally we would specify this conversion as an impl of `ImplicitAs` in
the prelude instead of hardcoding it in the interop layer, but
unfortunately that's not currently possible, for various reasons -- we
can't make the conversion form-generic, we can't deduce the array length
from the initializer, and we can't deduce against the arguments of
imported C++ class templates yet -- so for now synthesizing a builtin
function on demand is the best we can do.

Assisted-by: Gemini 3 Pro via Antigravity

toolchain/sem_ir/cpp_initializer_list.cpp

@@ -0,0 +1,54 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/sem_ir/cpp_initializer_list.h"
+
+#include "toolchain/sem_ir/file.h"
+#include "toolchain/sem_ir/inst_categories.h"
+#include "toolchain/sem_ir/typed_insts.h"
+
+namespace Carbon::SemIR {
+
+auto GetStdInitializerListLayout(const File& sem_ir, TypeId type_id)
+    -> StdInitializerListLayout {
+  auto repr_id = sem_ir.types().GetObjectRepr(type_id);
+  if (!repr_id.has_value()) {
+    // An incomplete type doesn't have a recognized layout.
+    return StdInitializerListLayout{};
+  }
+
+  auto struct_type = sem_ir.types().TryGetAs<AnyStructType>(repr_id);
+  if (!struct_type) {
+    return StdInitializerListLayout{};
+  }
+
+  auto fields = sem_ir.struct_type_fields().Get(struct_type->fields_id);
+  if (fields.size() != 2) {
+    return StdInitializerListLayout{};
+  }
+
+  // The first field must be a pointer.
+  auto field0_type_id = sem_ir.types().GetTransitiveUnqualifiedAdaptedType(
+      sem_ir.types().GetTypeIdForTypeInstId(fields[0].type_inst_id));
+  if (!sem_ir.types().Is<PointerType>(field0_type_id.first)) {
+    return StdInitializerListLayout{};
+  }
+
+  // The second field can be either a pointer or an integer.
+  auto field1_type_id = sem_ir.types().GetTransitiveUnqualifiedAdaptedType(
+      sem_ir.types().GetTypeIdForTypeInstId(fields[1].type_inst_id));
+  if (sem_ir.types().Is<PointerType>(field1_type_id.first)) {
+    return StdInitializerListLayout{
+        .kind = StdInitializerListLayout::PointerPointer};
+  }
+  if (sem_ir.types().TryGetIntTypeInfo(field1_type_id.first)) {
+    return StdInitializerListLayout{
+        .kind = StdInitializerListLayout::PointerInt,
+        .size_type_id = field1_type_id.first};
+  }
+
+  return StdInitializerListLayout{};
+}
+
+}  // namespace Carbon::SemIR