Add support for C++ calling Carbon functions with parameters (#7024)

This works by generating two thunks, one in C++ and one in Carbon. For
example, given this input:
```c++
// Carbon:
fn Callme(f: f32) {}

// C++:
void F() {
  // This will call `Callme__cpp_thunk`
  Carbon::Callme(1.0);
}
```

These functions are generated:
```c++
// Carbon:
fn Callme__carbon_thunk(ref f: f32) {
  // Call the target function.
  Callme(f);
}

// C++:

// C++ declaration for the Carbon thunk.
void Callme__carbon_thunk(float& f);

void Callme__cpp_thunk(float f) {
  // Call the Carbon thunk with args passed by reference.
  Callme__carbon_thunk(f);
}
```

For now, all arguments are passed by reference, even if they are simple
types like pointers or i32.

Functions with non-void return types are not supported yet.

toolchain/check/cpp/generate_ast.cpp

@@ -28,6 +28,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/context.h"
+#include "toolchain/check/cpp/export.h"
 #include "toolchain/check/cpp/import.h"
 #include "toolchain/check/import_ref.h"
 #include "toolchain/check/name_lookup.h"
@@ -36,7 +37,6 @@
 #include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/cpp_file.h"
-#include "toolchain/sem_ir/mangler.h"
 
 namespace Carbon::Check {
 
@@ -432,56 +432,17 @@ auto CarbonExternalASTSource::MapInstIdToClangDecl(
       if (!callee_function) {
         return nullptr;
       }
+
       const SemIR::Function& function =
           context_->functions().Get(callee_function->function_id);
       if (function.clang_decl_id.has_value()) {
         return cast<clang::NamedDecl>(
             context_->clang_decls().Get(function.clang_decl_id).key.decl);
       }
-      auto* identifier_info =
-          GetClangIdentifierInfo(*context_, function.name_id);
-      CARBON_CHECK(identifier_info, "function with non-identifier name {0}",
-                   function.name_id);
 
-      if (function.call_param_ranges.explicit_size() != 0) {
-        context_->TODO(target_inst_id,
-                       "unsupported: C++ calling a Carbon function with "
-                       "parameters");
-        return nullptr;
-      }
-
-      if (function.return_type_inst_id != SemIR::TypeInstId::None) {
-        context_->TODO(target_inst_id,
-                       "unsupported: C++ calling a Carbon function with "
-                       "return type other than `()`");
-        return nullptr;
-      }
-
-      // TODO: support non-empty parameter lists.
-      llvm::SmallVector<clang::QualType> cpp_param_types;
-
-      // TODO: support non-void return types.
-      auto cpp_return_type = ast_context_->VoidTy;
-
-      auto cpp_function_type = ast_context_->getFunctionType(
-          cpp_return_type, cpp_param_types,
-          clang::FunctionProtoType::ExtProtoInfo());
-
-      auto* function_decl = clang::FunctionDecl::Create(
-          *ast_context_, &decl_context,
-          /*StartLoc=*/clang::SourceLocation(),
-          /*NLoc=*/clang::SourceLocation(),
-          clang::DeclarationName(identifier_info), cpp_function_type,
-          /*TInfo=*/nullptr, clang::SC_Extern);
-
-      // Mangle the function name and attach it to the `FunctionDecl`.
-      SemIR::Mangler m(context_->sem_ir(), context_->total_ir_count());
-      std::string mangled_name =
-          m.Mangle(callee_function->function_id, SemIR::SpecificId::None);
-      function_decl->addAttr(
-          clang::AsmLabelAttr::Create(*ast_context_, mangled_name));
-
-      return function_decl;
+      return GetReverseInteropFunctionDecl(
+          *context_, SemIR::LocId(target_inst_id), decl_context,
+          callee_function->function_id);
     }
     default:
       return nullptr;