Change Context::IsImplFile to File::is_impl (#4931)

Note this mirrors parse_tree().packaging_decl().is_impl, but I'm
preferring to keep the version that doesn't access the parse tree so
that we could change tree storage without hurting as much.

toolchain/check/handle_class.cpp

@@ -10,6 +10,7 @@
 #include "toolchain/check/eval.h"
 #include "toolchain/check/generic.h"
 #include "toolchain/check/handle.h"
+#include "toolchain/check/import.h"
 #include "toolchain/check/import_ref.h"
 #include "toolchain/check/merge.h"
 #include "toolchain/check/modifiers.h"
@@ -265,7 +266,7 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
                          .specific_id = specific_id}));
   }
 
-  if (!is_definition && context.IsImplFile() && !is_extern) {
+  if (!is_definition && context.sem_ir().is_impl() && !is_extern) {
     context.definitions_required().push_back(class_decl_id);
   }
 
@@ -333,7 +334,8 @@ static auto DiagnoseClassSpecificDeclOutsideClass(Context& context,
 static auto GetCurrentScopeAsClassOrDiagnose(Context& context, SemIRLoc loc,
                                              Lex::TokenKind tok)
     -> std::optional<SemIR::ClassDecl> {
-  auto class_scope = context.GetCurrentScopeAs<SemIR::ClassDecl>();
+  auto class_scope =
+      context.scope_stack().GetCurrentScopeAs<SemIR::ClassDecl>();
   if (!class_scope) {
     DiagnoseClassSpecificDeclOutsideClass(context, loc, tok);
   }