ir: annotate refutable pattern bidings as @unchecked

Beginning in Scala 3.2, refutable pattern bindings will warn by default,
see https://dotty.epfl.ch/docs/reference/changed-features/pattern-bindings.html

Since the scalajs-ir code is unpacked and recompiled from source as
part of the Scala 3 build, which uses -Xfatal-warnings, we suppress
the warning here with an @unchecked annotation to unblock the dotty
PR (scala/scala3#14294) introducing this behavioral change.

This commit does not address such usage in any other portion of the
codebase, which may need to be addressed at a future date.

Author: griggt

ir/shared/src/main/scala/org/scalajs/ir/Trees.scala

@@ -1203,7 +1203,7 @@ object Trees {
       case TopLevelJSClassExportDef(_, name) => name
 
       case TopLevelMethodExportDef(_, JSMethodDef(_, propName, _, _, _)) =>
-        val StringLiteral(name) = propName
+        val StringLiteral(name) = propName: @unchecked  // unchecked is needed for Scala 3.2+
         name
 
       case TopLevelFieldExportDef(_, name, _) => name