Skip ro segments which are not in range when walking heap for GC diagnostics

[alexey-zakharov]

Fixed crash in gc_heap::walk_heap_per_heap when walking frozen segments

Motivation:

GC Heap diagnostics crashes after AssemblyLoadContext unloading with the following callstack

[Inline Frame] coreclr.dll!WKS::my_get_size(Object *) Line 11572	C++	Symbols loaded.
>	coreclr.dll!WKS::gc_heap::walk_heap_per_heap(bool(*)(Object *, void *) fn, void * context, int gen_number, int walk_large_object_heap_p) Line 51974	C++	Symbols loaded.
 	coreclr.dll!GCProfileWalkHeapWorker(int fProfilerPinned, int fShouldWalkHeapRootsForEtw, int fShouldWalkHeapObjectsForEtw) Line 727	C++	Symbols loaded.
 	coreclr.dll!GCToEEInterface::DiagGCEnd(unsigned __int64 fConcurrent, int index, int gen, bool reason) Line 818	C++	Symbols loaded.
 	coreclr.dll!WKS::gc_heap::do_post_gc() Line 50183	C++	Symbols loaded.
 	coreclr.dll!WKS::gc_heap::gc1() Line 22865	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!GCToOSInterface::GetLowPrecisionTimeStamp() Line 1091	C++	Symbols loaded.
 	coreclr.dll!WKS::gc_heap::garbage_collect(int n) Line 24329	C++	Symbols loaded.
 	coreclr.dll!WKS::GCHeap::GarbageCollectGeneration(unsigned int gen, gc_reason reason) Line 50526	C++	Symbols loaded.
 	coreclr.dll!WKS::GCHeap::GarbageCollect(int generation, bool low_memory_p, int mode) Line 49681	C++	Symbols loaded.
 	coreclr.dll!ETW::GCLog::ForceGCForDiagnostics() Line 492	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ETW::GCLog::ForceGC(__int64) Line 431	C++	

The issue can be reproduced with PerfView tool without Unity Editor enabling COR_PRF_MONITOR_GC flag at startup - tool crashes on GC Heap dump of Unity Editor after we successfully unloaded an AssemblyLoadContext.

I was suspecting heap corruption, but was not able to gather any proof. Comparison with gc::verify_heap yielded that heap verification uses heap_segment_in_range_p method to get the heap segment to iterate over. heap_segment_in_range_p checks that segment is not readonly/frozen (heap_segment_flags_readonly flag) or if it is “in range” (heap_segment_flags_inrange flag).

Changes:

It is a bit unclear to full extent what USE_RANGES feature do, however walking heap for diagnostics is different than Heap Verify functionality, so I suggest align segments walking strategy and ask upstream whether or not this is expected to be same or not.