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/cpp/generate_ast.cpp

@@ -86,34 +86,6 @@ static auto GenerateCppIncludesHeaderCode(
       }
     }
   }
-
-  // Inject a declaration of placement operator new, because the code we
-  // generate in thunks depends on it for placement new expressions. Clang has
-  // special-case logic for lowering a new-expression using this, so a
-  // definition is not required.
-  // TODO: This is a hack. We should be able to directly generate Clang AST to
-  // construct objects in-place without this.
-  // TODO: Once we can rely on libc++ being available, consider including
-  // `<__new/placement_new_delete.h>` instead.
-  code_stream << R"(# 1 "<carbon-internal>"
-#undef constexpr
-#if __cplusplus > 202302L
-constexpr
-#endif
-#undef void
-#undef operator
-#undef new
-void* operator new(__SIZE_TYPE__, void*)
-#if __cplusplus < 201103L
-#undef throw
-throw()
-#else
-#undef noexcept
-noexcept
-#endif
-;
-)";
-
   return code;
 }