Add Check::Context::clang_sema() method and use it (#6110)

This replaces `sem_ir().clang_ast_unit()->getSema()`.

toolchain/check/cpp/import.cpp

@@ -506,9 +506,7 @@ static auto GetDeclContext(Context& context, SemIR::NameScopeId scope_id)
 static auto ClangLookup(Context& context, SemIR::NameScopeId scope_id,
                         clang::DeclarationName name)
     -> std::optional<clang::LookupResult> {
-  clang::ASTUnit* ast = context.sem_ir().clang_ast_unit();
-  CARBON_CHECK(ast);
-  clang::Sema& sema = ast->getSema();
+  clang::Sema& sema = context.clang_sema();
 
   // TODO: Map the LocId of the lookup to a clang SourceLocation and provide it
   // here so that clang's diagnostics can point into the carbon code that uses
@@ -533,7 +531,7 @@ static auto ClangConstructorLookup(Context& context,
     -> clang::DeclContextLookupResult {
   const SemIR::NameScope& scope = context.name_scopes().Get(scope_id);
 
-  clang::Sema& sema = context.sem_ir().clang_ast_unit()->getSema();
+  clang::Sema& sema = context.clang_sema();
   clang::Decl* decl =
       context.clang_decls().Get(scope.clang_decl_context_id()).decl;
   return sema.LookupConstructors(cast<clang::CXXRecordDecl>(decl));
@@ -578,11 +576,8 @@ static auto GetDeclarationName(Context& context, SemIR::NameId name_id)
     return std::nullopt;
   }
 
-  return clang::DeclarationName(context.sem_ir()
-                                    .clang_ast_unit()
-                                    ->getSema()
-                                    .getPreprocessor()
-                                    .getIdentifierInfo(*name));
+  return clang::DeclarationName(
+      context.clang_sema().getPreprocessor().getIdentifierInfo(*name));
 }
 
 // Looks up the given name in the Clang AST in a specific scope. Returns the