[torque] improve enum generation for bitsets

Explicitly specify the enum base type for Flags enums generated
from Torque bitfield structs. Before, this was implicitly a signed
integer type. This caused a recent gcc compile issue with signed
and unsigned comparisons triggered by
https://chromium-review.googlesource.com/c/v8/v8/+/3251177

Bug: v8:7793
Change-Id: Iceb3c8632cfc95766b5e6ce7fae47cf5d002b9f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3253358
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77658}

src/torque/implementation-visitor.cc

@@ -3864,11 +3864,11 @@ void ImplementationVisitor::GenerateBitFields(
 
       // If every field is one bit, we can also generate a convenient enum.
       if (all_single_bits) {
-        header << "  enum Flag { \\\n";
+        header << "  enum Flag: " << type_name << " { \\\n";
         header << "    kNone = 0, \\\n";
         for (const auto& field : type->fields()) {
-          header << "    k" << CamelifyString(field.name_and_type.name)
-                 << " = 1 << " << field.offset << ", \\\n";
+          header << "    k" << CamelifyString(field.name_and_type.name) << " = "
+                 << type_name << "{1} << " << field.offset << ", \\\n";
         }
         header << "  }; \\\n";
         header << "  using Flags = base::Flags<Flag>; \\\n";