Only export each class or namespace to C++ once. (#7042)

Instead of exporting a class or namespace each time a new C++ name
lookup discovers it, track that we have exported the entity on its name
scope, and if a new name lookup finds the same entity, produce the same
clang declaration.

toolchain/check/cpp/export.h

@@ -11,11 +11,29 @@
 
 namespace Carbon::Check {
 
+// Exports a Carbon name scope into C++ as a namespace or class, or returns the
+// C++ namespace or class declaration that it was imported from.
+//
+// If the name scope has already been exported, returns the existing context.
+// Otherwise, creates a new C++ declaration context and returns it. Returns
+// nullptr if the name scope could not be exported and an error was diagnosed.
+auto ExportNameScopeToCpp(Context& context, SemIR::LocId loc_id,
+                          SemIR::NameScopeId name_scope_id)
+    -> clang::DeclContext*;
+
+// Exports a Carbon class into C++ as a class, or returns the C++ tag type that
+// the class was imported from.
+//
+// If the class has already been exported, returns the existing C++ class.
+// Otherwise, creates a new C++ class and returns it. Returns nullptr if the
+// class could not be exported and an error was diagnosed.
+auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
+                      SemIR::InstId class_inst_id, SemIR::ClassType class_type)
+    -> clang::TagDecl*;
+
 // Get a `clang::FunctionDecl` that can be used to call a Carbon function.
-auto GetReverseInteropFunctionDecl(Context& context, SemIR::LocId loc_id,
-                                   clang::DeclContext& decl_context,
-                                   SemIR::FunctionId function_id)
-    -> clang::FunctionDecl*;
+auto ExportFunctionToCpp(Context& context, SemIR::LocId loc_id,
+                         SemIR::FunctionId function_id) -> clang::FunctionDecl*;
 
 }  // namespace Carbon::Check