IL: reuse the cached ILTypeRef in ILTypeInfo.FromType

[auduchinok]

ILTypeInfo.FromType built its ILTypeRef with mkRefForNestedILTypeDef scoref (enc, tdef) on every
call. For a TILObjectRepr entity, Entity.CompiledRepresentation already caches the result of that
exact expression (TypedTree.fs), reachable as tcref.CompiledRepresentationForNamedType.

FromType is called once per MethInfo — MethInfo.CreateILMeth calls it for every method of a type —
so the uncached path produced one duplicate ILTypeRef per method info, each re-deriving the enclosing
name list and re-computing the hash. This takes the cached one instead, in both the tuple and the plain
isILAppTy branch.

In a heap snapshot of ParseAndCheckProject over a 57-file project with 489 references, ILTypeRef
went from 150,928 live objects (8.06 MB) to 53,262 (2.84 MB) — 98,398 of them were duplicates held
one-per-ILTypeInfo.

Project	Before (MB)	After (MB)	Diff
consoleapp	33.84	33.84	−0.00 (−0.01%)
Fantomas.Core	109.37	109.28	−0.09 (−0.08%)
Fantomas.Core.Tests	141.89	141.55	−0.34 (−0.24%)
Fantomas.Benchmarks	64.81	64.79	−0.02 (−0.04%)
FSharp.Common	297.61	292.54	−5.07 (−1.70%)
fcs	1271.39	1264.63	−6.76 (−0.53%)

[github-actions]

❗ Release notes required

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---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

[T-Gro]

Nice, surgical memory win — LGTM. ✅

I traced the equivalence to be sure the cached ref is identical, not just similar:

• Both branches are gated by isILAppTy/ILTyconInfo, which require IsILTycon, i.e. TypeReprInfo = TILObjectRepr. So the entity is always a TILObjectRepr.
• For exactly that shape, Entity.CompiledRepresentation (TypedTree.fs) computes mkRefForNestedILTypeDef ilScopeRef (ilEnclosingTypeDefs, ilTypeDef) — the same expression FromType was re-running with scoref (enc, tdef) — and CompiledRepresentationForNamedType returns that ILAsmNamed tref. Same value, now shared.
• The one thing that could have diverged is that CompiledRepresentation consults ExceptionInfo before TypeReprInfo, but TExnAbbrevRepr/TExnAsmRepr only arise for F# exception entities, which are never TILObjectRepr and so never reach FromType. No behavior change.

Since ILTypeRef equality/hash is structural, collapsing the ~98k per-ILTypeInfo duplicates into the shared cached instance is pure upside — the heap numbers back it up nicely, and FromType runs once per MethInfo so this was a real hot allocator.

Verified locally: FSharp.Compiler.Service builds clean for net11.0 and netstandard2.0 (only the pre-existing FS3261 in XmlDocInheritance.fs).

Tiny non-blocking nit: the note landed under ### Added — a memory optimization reads more like ### Improved, but not worth a spin.