Create a placement operator new directly. (#6941)

Instead of injecting code to declare an `operator new`, generate AST for
it directly. In order to use this, directly generate a `CXXNewExpr`
rather than asking Clang to build one.

This is less of a hack, and doesn't visibly leak an `operator new`
declaration that inline C++ code or template instantiations might see.
It also avoids generating a warning in C++26 and later that the
`constexpr` declaration of `operator new` is used but not defined.

Assisted-by: Gemini 3.1 Pro via Antigravity

toolchain/check/testdata/interop/cpp/function/thunk_cpp26.carbon

@@ -0,0 +1,28 @@
+// 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-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+// EXTRA-ARGS: --clang-arg=--std=c++26
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/thunk_cpp26.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/thunk_cpp26.carbon
+
+// --- thunk_required.h
+
+auto foo(short a) -> short;
+
+// --- import_thunk_required.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "thunk_required.h";
+
+fn F() {
+  // Placement operator new is `constexpr` in C++26. But that shouldn't cause
+  // problems for us because we declare our own operator new.
+  Cpp.foo(1 as i16);
+}