Add a flag to build a single ASTContext shared across all compilations (#7567)

Instead of building one Clang `ASTContext` per compilation, the
`--share-cpp-ast` flag causes us to build a single `ASTContext` and
share it across all contexts. One new abstraction is added: `CppDomain`
represents the Carbon-side view of a Clang AST that might be shared
across multiple `SemIR::File`s. This object owns the Clang instance and
the AST.

For now, we have no isolation between the C++ state exposed to different
Carbon compilations, and we have no multiplexing of generated LLVM IR
from C++ into different Carbon compilations, so the mode is not usable
yet. The plan is to keep it behind a flag until it's ready.

Assisted-by: Gemini via Antigravity

toolchain/check/cpp/import.cpp

@@ -118,9 +118,7 @@ static auto AddNamespace(Context& context, PackageNameId cpp_package_id,
 
 auto ImportCpp(Context& context,
                llvm::ArrayRef<Parse::Tree::PackagingNames> imports,
-               llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> fs,
-               llvm::LLVMContext* llvm_context,
-               std::shared_ptr<clang::CompilerInvocation> invocation) -> void {
+               CppDomain* domain) -> void {
   if (imports.empty()) {
     // TODO: Consider always having a (non-null) AST even if there are no Cpp
     // imports.
@@ -137,7 +135,7 @@ auto ImportCpp(Context& context,
   SemIR::NameScope& name_scope = context.name_scopes().Get(name_scope_id);
   name_scope.set_is_closed_import(true);
 
-  if (GenerateAst(context, imports, fs, llvm_context, std::move(invocation))) {
+  if (domain && GenerateAst(context, imports, *domain)) {
     name_scope.set_clang_decl_context_id(
         context.clang_decls().Add(
             {.key = SemIR::ClangDeclKey(