IL: hold custom attributes in fields rather than a union case

[auduchinok]

ILAttributesStored wrapped its state in a union:

type internal ILAttributesStoredRepr =
    | Reader of (int32 -> ILAttribute[])
    | Given of ILAttributes

so every attribute owner — type def, method, field, property, event, parameter, interface impl —
allocated a wrapper on top of the stored object itself. In a held FSharp.Common analysis 67,605 of 86,667
wrappers are Reader: 78% of owners never have their attributes read, and the wrapper existed only to
record that.

The change

type ILAttributesStored private (metadataIndex: int32, reader: int32 -> ILAttribute[], given: ILAttribute[] | null)

The reader function is shared per metadata reader per attribute table, so it can be held directly;
ILAttributes is a struct over the array, so rewrapping on each read allocates nothing. CreateGiven
stores the array with a shared noReader sentinel to keep the field non-null. Net per owner: −24 bytes of
wrapper, +8 for the second field. [<VolatileField>] is kept, and the race is the same benign one as
before — two threads may both run the reader and store equal results.

No API change: ILAttributesStoredRepr was internal and not in il.fsi; the members keep their
signatures and the union type is deleted.

Measured

Against 90767c77e9 built in the same session. er-base2 is a byte-identical copy of that baseline run
through the whole harness, so its column is the measurement error; a delta not exceeding both it and the
baseline's own spread is marked not established.

One project held open (mean of 3 fresh processes)

Project	Before	After	Retained
consoleapp	33.84 MB	33.57 MB	-0.27 MB (-0.79%)
Oxpecker	72.83 MB	72.27 MB	-0.56 MB (-0.77%)
IcedTasks	50.14 MB	49.94 MB	-0.20 MB (-0.39%)
FsToolkit	75.39 MB	75.20 MB	-0.19 MB (-0.25%)
Fantomas.Benchmarks	64.77 MB	64.57 MB	-0.20 MB (-0.31%)
Prime	108.37 MB	108.07 MB	-0.30 MB (-0.28%)
Fantomas.Core	109.50 MB	109.27 MB	-0.24 MB (not established)
Fantomas.Core.Tests	141.95 MB	141.65 MB	-0.31 MB (-0.22%)
FSharp.Common	297.67 MB	296.35 MB	-1.32 MB (-0.45%)
fcs	1273.30 MB	1271.81 MB	-1.48 MB (not established)

A whole solution held open at once (mean of 2 passes)

Solution	Before	After	Retained
resharper	1746.27 MB	1743.97 MB	-2.29 MB (-0.13%)
fcsrepo	2557.91 MB	2554.06 MB	-3.85 MB (-0.15%)
fantomas	690.18 MB	689.26 MB	-0.92 MB (-0.13%)
oxpecker	254.06 MB	253.00 MB	-1.06 MB (-0.42%)
fstoolkit	238.00 MB	237.56 MB	-0.44 MB (not established)
icedtasks	212.48 MB	211.87 MB	-0.61 MB (-0.28%)
prime	231.86 MB	231.39 MB	-0.47 MB (-0.20%)
consoleapp	33.82 MB	33.62 MB	-0.20 MB (-0.60%)

What it removes

gcdump of one held FSharp.Common analysis: Reader 67,605 → 1,467, Given 19,062 → 0 — 1.99 MB of
wrappers — against ILAttributesStored growing 2.60 → 3.25 MB for the second field. Net 1.34 MB predicted,
1.32 MB measured. The 1,467 remaining Reader objects are ILSecurityDeclsStored's own union, untouched
here.

[github-actions]

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