Fix #3008: local function forwarding a display class loses its scope

[siegfriedpammer]

Written by Claude (agent), posted on behalf of @siegfriedpammer.

Fixes #3008.

Decompiling Microsoft.CodeAnalysis.CSharp.Binder.CreateConversion asserted at Debug.Assert(localFunction != null) in CallBuilder, and the Release build emitted uncompilable code - calls written as raw metadata names, <CreateConversion>g__reportUseSiteDiagnostics|253_1(...).

DetermineCaptureAndDeclarationScope derives a local function's declaration scope from the display-class variable at its use sites. Roslyn passes display classes into local functions as ref parameters, and GetClosureInitializer returns null for a parameter, so a local function used only from inside another local function never resolved a scope. It fell through to the blanket root-body fallback and was hoisted out of the function whose closure its callees read, leaving those callees out of scope. A closure parameter now resolves to the variable that actually holds the display class, so all three end up siblings in the same local function, matching the source layout.

The constructor branch had a second, smaller instance of the same problem: use sites in separate function bodies share no BlockContainer, so FindCommonAncestorInstruction returned null and a valid closure-derived scope was discarded. The use-site scope is now computed separately and the closure-derived one kept when there is none.

Two points for review:

• The constructor-path change makes the use-site scope win outright instead of folding it into the closure scope. The suite is green either way; it is the one judgement call in the diff.
• An alternative fix - declaring at the innermost function containing all use sites - also resolved this case but tripped TryValidateSkipCount's assert on Generic<T1>.MixedLocalFunction, which assumes generic local functions are declared at the root. Worth knowing if anyone revisits this area.

Pretty fixture Issue3008_DisplayClassForwardedThroughLocalFunction covers the shape. A whole-assembly Debug decompile of the reporter's assembly no longer asserts.