Fix: warn FS0049 on upper union case labels

[edgarfgp]

Description

This PR fixes a bug where match clauses without the optional leading bar (|) were being treated differently from those with it.

The IsTrueMatchClause property was checking for both a bar and an arrow, but the bar is actually optional syntax in F#. This meant these two equivalent patterns behaved differently:

// This worked fine
match x with
| SomeCase -> 1

// This had issues  
match x with SomeCase -> 1

The same problem affected function expressions and try...with blocks when the first clause didn't have a bar.

This only affects SynMatchClause nodes in the AST (used by match, function, and try...with).
Not other uses of -> in F# like type signatures or lambdas.

Looking at the parser, ArrowRange is only None when we're in error recovery or dealing with compiler-generated synthetic clauses. user written match clauses always have an arrow, regardless of whether they have a bar.

Fixes #18992

Checklist

• Test cases added
• Release notes entry updated

[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

[T-Gro]

When it comes to the known regression this fixes, it is just the uppercase warning?
It is very late for the .NET10 (GA) process, and I need to weight in the fix against the late notice insertion.

If it is just the warning, I would go with this fix in main only, and keep .NET10 GA as is.
(since it is a warning and does have a workaround)