[CSSyntaticElement] Canonicalize type before collecting "in scope" variables

Follow-up to #65048

`getDesugaredType` unwraps sugar types that appear in sequence,
to remove sugar from nested positions we need to get a canonical type.

Thanks to @slavapestov for pointing it out.

Author: xedin

lib/Sema/CSSyntacticElement.cpp

@@ -217,7 +217,7 @@ class TypeVariableRefFinder : public ASTWalker {
     // For example `Typealias<$T, $U>.Context` which desugars into
     // `_Context<$U>` would bring in `$T` that could be inferrable
     // only after the body of the closure is solved.
-    type = type->getDesugaredType();
+    type = type->getCanonicalType();
 
     // Don't walk into the opaque archetypes because they are not
     // transparent in this context - `some P` could reference a