Treat span-like types with a unit of measure as byref-like #20907

A measure-annotated abbreviation of Span<int> is not treated as byref-like when its unit of measure is anything other than 1. The compiler then accepts code that captures such a value in a closure or uses its type as a type argument, and the program fails at run time.

open System
[<Measure>] type kg
[<MeasureAnnotatedAbbreviation>]
type TaggedSpan<[<Measure>] 'u> = Span<int>
let size (s: TaggedSpan<'u>) = 0

let capture (s: TaggedSpan<kg>) = let t = s in fun () -> size t
let instantiate (s: TaggedSpan<kg>) = id s

[<EntryPoint>]
let main args =
    let s = Unchecked.defaultof<TaggedSpan<kg>>
    if args.Length = 0 then capture s () else size (instantiate s)

Actual: this compiles with the .NET SDK 10.0.402 compiler and with main at fbc9be3. Compiled without optimization:

• With no arguments, capture throws TypeLoadException: A ByRef or ByRef-like type cannot be used as the type for an instance field in a non-ByRef-like type.
• With an argument, instantiate throws VerificationException: Method Microsoft.FSharp.Core.Operators.Identity: type argument 'System.Span`1[System.Int32]' violates the constraint of type parameter 'T'.

Expected: the errors the compiler reports when kg is replaced by 1: FS0406 on the closure in capture, and FS0412 on id s and on Unchecked.defaultof.

isByrefLikeTyconRef does not look at the representation of a measure-annotated type, and stripTyEqns reduces such a type to its representation only when the measure is dimensionless, so TaggedSpan<kg> is not recognised as byref-like. I found this while reviewing #20856, whose tests define TaggedSpan. I have a fix ready.