Fix a warning and an uninitialized variable in benchmark (#200927)

After #198964 I see a -Wcovered-switch-default warning in sysinfo.cc,
but it looks like it's uncovering something a bit worse.

Back in #147357 and #149159 it looks like we tried to fix an MSVC
warning for an uncovered `CacheUnknown` case in this switch, and removed
the initialization of the variable before the switch. The update to
Google Benchmark v1.9.5 has a different fix for this - it's handling
that specific case guarded by some MSVC version macros, but this depends
on the initialization we removed!

Add the initialization back and remove the default case, effectively
reverting #149159 and #147357.

third-party/benchmark/src/sysinfo.cc

@@ -370,6 +370,7 @@ std::vector<CPUInfo::CacheInfo> GetCacheSizesWindows() {
     C.num_sharing = static_cast<int>(b.count());
     C.level = cache.Level;
     C.size = static_cast<int>(cache.Size);
+    C.type = "Unknown";
     switch (cache.Type) {
 // Windows SDK version >= 10.0.26100.0
 #ifdef NTDDI_WIN11_GE
@@ -388,9 +389,6 @@ std::vector<CPUInfo::CacheInfo> GetCacheSizesWindows() {
       case CacheTrace:
         C.type = "Trace";
         break;
-      default:
-        C.type = "Unknown";
-        break;
     }
     res.push_back(C);
   }