Cache hash code in TypeStructure to fix O(n) GetHashCode perf regression

[OnurGumus]

Summary

TypeStructure.GetHashCode calls GenericHashArbArray on the underlying TypeToken[] on every cache lookup, making each hash computation O(n) over the token array. In IDE mode with generative type providers, the type subsumption cache continuously rehashes all entries during rebuildStore, causing sustained ~150% CPU.

This PR pre-computes the hash when creating TypeStructure and stores it in the DU case, making GetHashCode O(1). It also adds [<CustomEquality; NoComparison>] with a fast-path Equals that checks the cached hash before comparing arrays.

Background

A 10-second dotnet-trace CPU sample on the FSAC process (166% CPU) with a solution using generative type providers shows:

Samples	Function
2414	Thread.PollGC
1170	RuntimeTypeHandle.InternalAllocNoChecks
820	GenericEqualityArbArray
696	TTypeCacheKey.GetHashCode → GenericHashArbArray
96	ConcurrentDictionary.GrowTable
12	Cache.rebuildStore

PR #18926 memoized TypeStructure creation via a ConditionalWeakTable, but GetHashCode still delegates to F# structural hashing on ImmutableArray<TypeToken> (which wraps T[]), making each call O(n). The cache's rebuildStore re-hashes all existing entries, amplifying the cost.

Repro steps

1. Create an F# solution targeting net10.0 (LangVersion 10) with 5+ projects.
2. Add a generative type provider producing ~15 provided types.
3. Reference the type provider from 2+ projects in the solution.
4. Open the solution in VS Code with Ionide (FSAC).
5. Observe sustained ~150% CPU on the dotnet fsautocomplete.dll process that does not settle down.

Setting <LangVersion>9.0</LangVersion> (disabling UseTypeSubsumptionCache) eliminates the CPU issue entirely.

Changes

• src/Compiler/Utilities/TypeHashing.fs: Added hashTokenArray helper. Changed TypeStructure DU cases to carry a pre-computed hash: int. Added [<CustomEquality; NoComparison>] with O(1) GetHashCode and fast-path Equals.
• src/Compiler/Checking/OverloadResolutionCache.fs: Updated pattern matches for new Stable(hash, tokens) / Unstable(hash, tokens) shape.

Related

• PR Type subsumption cache, fix keys to reduce memory use #18875 (type subsumption cache introduction)
• Issue #18875 degraded typecheck performance #18925
• PR Fix type subsumption cache key perf regression #18926 (partial fix — memoized TypeStructure creation, not hash)
• Issue Performance Regression: Build takes 7s in .net9 and 5min in .net10 #19169 (build regression)

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[majocha]

Lots of calls to Cache.rebuildStore indicate a serious underlying problem.