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/context.cpp

@@ -6,11 +6,10 @@
 
 #include "clang/AST/Mangle.h"
 #include "clang/Frontend/CompilerInstance.h"
-#include "toolchain/check/cpp/domain.h"
 
 namespace Carbon::Check {
 
-CppContext::CppContext(CppDomain& domain,
+CppContext::CppContext(SemIR::CppDomain& domain,
                        std::unique_ptr<CppDiagnosticListener> listener)
     : domain_(&domain), diagnostic_listener_(std::move(listener)) {}