[cDAC] Fix IsSharedByGenericInstantiations for non-shared canonical MTs

[max-charlamb]

The cDAC's IsSharedByGenericInstantiations fell back to IsCanonMT && HasInstantiation, which is too permissive: a value-type generic instantiation like ArraySortHelper<int> is its own canonical MT (value-type instantiations don't share with __Canon), so the check returned true and GetGenericContextLoc reported InstArgMethodTable for methods that don't actually need an inst arg.

This mirrors the runtime's MethodTable::IsSharedByGenericInstantiations (methodtable.inl:1044): GenericsMask bits == enum_flag_GenericsMask_SharedInst. The cDAC's MethodTableFlags_1 was missing the SharedInst (0x20) value of the GenericsMask bits; this adds it (the GenericInst 0x10 value is not used by the contract so left out per usual cDAC convention) and uses it directly.

Methods now correctly classified (previously reported a spurious TYPE_PARAM):

• ArraySortHelper<int>.cctor() / .CreateArraySortHelper()
• ArraySortHelper<ulong>.cctor() / .CreateArraySortHelper()
• ReadOnlySpan<char>.ToString()
• Dictionary.CollectionsMarshalHelper.GetValueRefOrAddDefault(...)

Also updates docs/design/datacontracts/RuntimeTypeSystem.md to reflect the new flag value and the simplified implementation, and the [cDAC] [RuntimeTypeSystem] dependency comment in src/coreclr/vm/methodtable.h.

Surfaced by the cdacstress ArgIterator sub-check (DOTNET_CdacStress=0x201).

Note

This PR description was authored with assistance from GitHub Copilot.

[hoyosjs]

From a bug fix perspective, makes sense

[Copilot]

Pull request overview

This PR corrects the cDAC RuntimeTypeSystem contract’s notion of “shared by generic instantiations” by using the runtime’s GenericsMask == SharedInst bit pattern instead of the broader IsCanonMT && HasInstantiation heuristic, and aligns documentation/comments with the updated contract flag value.

Changes:

• Add GenericsMask_SharedInst (0x20) to the contract’s MethodTableFlags_1 and expose IsSharedByGenericInstantiations.
• Update RuntimeTypeSystem_1.IsSharedByGenericInstantiations to use the new flag-derived property.
• Update RuntimeTypeSystem contract documentation and the CoreCLR dependency comment to reflect the new flag value.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/RuntimeTypeSystemHelpers/MethodTableFlags_1.cs	Adds GenericsMask_SharedInst and a corresponding IsSharedByGenericInstantiations helper to match runtime semantics.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/RuntimeTypeSystem_1.cs	Switches shared-generic detection to the new flag-based check for correct GetGenericContextLoc classification.
src/coreclr/vm/methodtable.h	Updates the [cDAC] [RuntimeTypeSystem] dependency comment to include SharedInst.
docs/design/datacontracts/RuntimeTypeSystem.md	Documents the new flag value and the simplified shared-generic detection logic.

[max-charlamb]

/ba-g known issue: #129705

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