Create modules for header imports. (#7613)

Make multiple imports of the same header only parse it once per C++
domain. Reuse of the same header in `--share-cpp-ast` mode now reuses
the representation.

Importing a Carbon file with C++ dependencies now makes those transitive
C++ dependencies in the same C++ domain visible too.

Assisted-by: Gemini via Antigravity

toolchain/check/cpp/import.cpp

@@ -39,7 +39,6 @@
 #include "toolchain/check/core_identifier.h"
 #include "toolchain/check/cpp/access.h"
 #include "toolchain/check/cpp/custom_type_mapping.h"
-#include "toolchain/check/cpp/domain.h"
 #include "toolchain/check/cpp/generate_ast.h"
 #include "toolchain/check/cpp/location.h"
 #include "toolchain/check/cpp/macros.h"
@@ -62,6 +61,7 @@
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/clang_decl.h"
 #include "toolchain/sem_ir/class.h"
+#include "toolchain/sem_ir/cpp_domain.h"
 #include "toolchain/sem_ir/cpp_file.h"
 #include "toolchain/sem_ir/cpp_overload_set.h"
 #include "toolchain/sem_ir/function.h"
@@ -119,7 +119,7 @@ static auto AddNamespace(Context& context, PackageNameId cpp_package_id,
 
 auto ImportCpp(Context& context,
                llvm::ArrayRef<Parse::Tree::PackagingNames> imports,
-               CppDomain* domain) -> void {
+               SemIR::CppDomain* domain) -> void {
   if (imports.empty()) {
     // TODO: Consider always having a (non-null) AST even if there are no Cpp
     // imports.
@@ -260,6 +260,22 @@ auto FindCorrespondingClangDeclKey(Context& context, SemIR::LocId loc_id,
   }
   CARBON_CHECK(clang_decl_id.has_value());
   auto key = file.clang_decls().Get(clang_decl_id).key;
+
+  // Easy case: files are from the same domain. We can just reuse the decl
+  // pointer.
+  if (context.sem_ir().cpp_file() && file.cpp_file() &&
+      context.sem_ir().cpp_file()->cpp_domain() ==
+          file.cpp_file()->cpp_domain()) {
+    if (key.signature_id.has_value()) {
+      key.signature_id = context.sem_ir().clang_decl_signatures().Add(
+          file.clang_decl_signatures().Get(key.signature_id));
+    }
+    if (ImportCppDecl(context, loc_id, key) != SemIR::ErrorInst::InstId) {
+      return key;
+    }
+    return std::nullopt;
+  }
+
   const auto* decl = key.decl;
   auto* corresponding = FindCorrespondingDecl(context, loc_id, decl);
   if (!corresponding) {