Acquire type-loading completion flags in MethodTable and TypeDesc

[Copilot]

Plain readiness reads can observe completed type loading before its payload writes on Arm64, allowing a child type to retain null inherited dictionary pointers.

• MethodTable: Centralize an acquire snapshot using VolatileLoad for GetLoadLevel, IsFullyLoaded, and HasApproxParent.
• TypeDesc: Use an acquire snapshot for GetLoadLevel and IsFullyLoaded, with one flags read per load-level query.
• Scope: Preserve interlocked writers, flag values, layouts, and loader locking. No new tests.

• Fixes Concurrent generic type loading can leave inherited dictionaries null on Arm64 #134245

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[Copilot]

Copilot review overview

🔵 Needs a closer look

The low-level concurrency fix appears sound but lacks targeted validation on weakly ordered architectures.

Review effort: Balanced
Findings: None

What changed in this PR

Adds acquire semantics to CoreCLR type-loading completion checks, preventing stale payload reads on weakly ordered architectures.

Changes:

• Uses VolatileLoad for MethodTable readiness flags.
• Applies equivalent semantics to TypeDesc.
• Reuses one acquired snapshot per load-level query.

File	Description
src/​coreclr/​vm/​methodtable.h	Acquires MethodTable loading flags.
src/​coreclr/​vm/​typedesc.h	Acquires fully-loaded state.
src/​coreclr/​vm/​typedesc.cpp	Acquires one load-level snapshot.