Merge duplicate .cctors for generic unions with nullary cases and static bindings (#19445)

[edgarfgp]

Problem

A generic union with a nullary case crashes the compiler when it also has static member val or static let bindings.

Fixes #19445.

Before

type U<'T> =
    | A
    static member val X = 3
// error FS0192: internal error: Error in pass2 for type U`1, error: duplicate entry '.cctor' in method table

type V<'T> =
    | B
    static let mutable counter = 1
    static member Next() =
        counter <- counter + 1
        counter
// same error

After

type U<'T> =
    | A
    static member val X = 3
// compiles; U<int>.X = 3, matching on U<int>.A works
// V<int>.Next() = 2, then 3; V<string>.Next() = 2 (statics are per instantiation)

Cause

Two code paths each contribute a .cctor to the union type: union erasure emits one initializing the nullary-case singleton field (_unique_A), and the static bindings of a generic type are compiled into a .cctor member for a non-generic type they run from the file initializer instead, which is why type U = A with static member val already worked. TypeDefBuilder.AddMethodDef appended the second one and ilwrite rejected the duplicate.

The two are now merged into the single .cctor IL permits: the straight-line singleton initializer (no locals, no handlers, no control flow) is prepended into the other initializer, so the singleton is initialized before user static bindings run the same order the non-generic path produces, and required because a binding may read the singleton:

type U<'T> =
    | A
    static member val X : U<'T> = A   // X is the initialized A singleton of the same instantiation

The merged initializer passes ILVerify, and no previously-compiling code changes shape the merge only happens where compilation used to fail.

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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