Support regions

[cshung]

This PR fixes #936.

Here are a few key changes in the heap data structure:

To get our terminologies straight, while we still have the heap_segment objects, we call them region objects under the context of USE_REGIONS.

Under USE_REGIONS, the heap_segment object itself is stored somewhere other than the address it manages (it is part of the seg_mapping_table to be precise), therefore we cannot use the address of the heap_segment object itself to determine the base address of the region.

With USE_REGIONS, gen0 and gen1 are no longer restricted on ephemeral_heap_segment, we could have a linked list of regions for gen0 and gen1. This explains the extra calls to AddSegments on ProcessHeap.

The field allocation_start is no longer defined in generation. For backward compatibility reasons, the field is still defined on the DacpGenerationData structure, but it will always be 0 in .NET 6 when USE_REGIONS is turned on. There isn't a single start and end pointer for gen0 and gen1 anymore. Instead, each region belongs to a generation, therefore the Gen0Start and Gen0Length are simply set to the Start and End-Start of the region if it is indeed a gen0 region, otherwise it is set to End and 0 respectively.

Beyond the obvious changes, here are some not so obvious changes that might break things:

1. The regions are no longer sorted, so I have to remove the check in the unit tests.
2. There could be empty regions, so I have to avoid walking them in heap enumeration.
3. By default, each region is just 4MB (LOH regions has a default size of 32MB), so one might have a lot more regions.

Status of the PR:

Right now this fix is sufficient to get the DumpGen command in SOS to work with dotnet/diagnostics#2501. With some hacks, all the unit tests passed. I might have missed other scenarios that are not covered by the tests.

Future work:

As the change on SOS side and unit test changes indicate, even when the fix is in, it is still possible for callers to be broken by USE_REGIONS. While there is nothing we can do in the code to prevent that, we might be able to communicate upfront to get it fixed earlier.

Eventually, we need to publish a new version with region support enabled so that SOS (and other tools) can consume the new functionalities.