Fix crash if a member of std::string_view is found in a derived class. (#6604)

Members of `std::string_view` can't be accessed directly, because that
type maps into Carbon's `str` type (`Core.String`), so member access
doesn't find the C++ members. But they can be named via qualified name
lookup into a derived type. That crashed because we didn't expect the
non-Cpp type `Core.String` to be the parent of a Cpp-imported member.

Plus add some more test coverage for related cases (not involving `str`)
that already worked.

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/check/cpp/import.cpp

@@ -254,9 +254,20 @@ static auto GetParentNameScopeId(Context& context, clang::Decl* clang_decl)
     auto class_inst_id =
         LookupClangDeclInstId(context, SemIR::ClangDeclKey(tag_decl));
     CARBON_CHECK(class_inst_id.has_value());
-    return context.classes()
-        .Get(context.insts().GetAs<SemIR::ClassDecl>(class_inst_id).class_id)
-        .scope_id;
+    auto class_inst = context.insts().Get(class_inst_id);
+    auto class_id = SemIR::ClassId::None;
+    if (auto class_decl = class_inst.TryAs<SemIR::ClassDecl>()) {
+      // Common case: the tag was imported as a new Carbon class.
+      class_id = class_decl->class_id;
+    } else {
+      // Rare case: the tag was imported as an existing Carbon class. This
+      // happens for C++ classes that get mapped to Carbon prelude types, such
+      // as `std::string_view`.
+      // TODO: In this case, should we import the C++ class declaration and use
+      // it as the parent, rather than using the existing Carbon class?
+      class_id = class_inst.As<SemIR::ClassType>().class_id;
+    }
+    return context.classes().Get(class_id).scope_id;
   }
 
   if (isa<clang::NamespaceDecl, clang::TranslationUnitDecl>(parent_decl)) {