Fix process background segment end (#53879)

Fix issue found in GetAllocatedBytesForCurrentThread with regions enabled.

The issue was traced to a region that had become empty in a background GC, but was still considered valid in the next ephemeral GC, so because there were cards set, we marked objects referenced from dead objects in the segment.

When a segment becomes completely empty in background GC, but is the start segment for the generation, we don't delete it, so we still need to reset heap_segment_allocated and decommit pages.

src/coreclr/gc/gc.cpp

@@ -38465,7 +38465,7 @@ void gc_heap::process_background_segment_end (heap_segment* seg,
                         heap_segment_mem (seg), (*delete_p ? "should" : "should not")));
 
         }
-        else
+        if (!*delete_p)
         {
             dprintf (3, ("[h%d] seg %Ix alloc %Ix->%Ix",
                 heap_number, (size_t)seg,