Syntax Tree: fix return type info for let! / and! / use!

[DedSec256]

Now there are several ways to annotate the binding:

• Annotate the head pattern

let (x: int) = 1

Tree:

SynBinding(
    ...
    headPat = SynPat.Paren(
        pat = SynPat.Typed(
            pat = SynPat.Named(ident = SynIdent.SynIdent(ident = Ident("x")),
            targetType = SynType.LongIdent(SynLongIdent.SynLongIdent(id = [ Ident("int") ]))
        ),
    ),
    returnInfo = None
)

• Annotate the binding return type

let x: int = 1

Tree:

SynBinding(
    headPat = SynPat.Named(ident = SynIdent.SynIdent(ident = Ident("x"))),
    returnInfo =
        Some(
            SynBindingReturnInfo.SynBindingReturnInfo(
                typeName = SynType.LongIdent(SynLongIdent.SynLongIdent(id = [ Ident("int") ]))
            )
        )
)

However, for annotated let! / and! / use! bindings the SynBindingReturnInfo is always None. Instead, a typed head pat is always produced as a part of the syntax tree, even if there was no annotated pattern in a source code.

This is inconsistent with simple let bindings and can create problems when handling typed let! / and! / use! binding

let! x: int = ...

in logic that relies on return type info.

This PR brings the syntax tree to a consistent view.

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

[edgarfgp]

Thanks @DedSec256 .

[DedSec256]

It seems that the fix in the tree is not enough. Types in the new syntax are not checked.
#19005