Fix compilation failures when resolving intrinsic sizeOf for Nat digi…

…ts in Scala 3. (#3245)

When using intrinsic version of sizeOf / alignmentOf for types using unsafe.Nat types in the projects in which they're defined (nativelib) it might happend that they're not yet available. In such case use fallback method of parsing digits based on class name

nscplugin/src/main/scala-3/scala/scalanative/nscplugin/NirGenType.scala

@@ -11,6 +11,7 @@ import core.Types._
 import core.Symbols._
 import core.StdNames._
 import core.TypeErasure._
+import core.TypeError
 import dotty.tools.dotc.report
 import dotty.tools.dotc.typer.TyperPhase
 import dotty.tools.dotc.transform.SymUtils._
@@ -237,11 +238,22 @@ trait NirGenType(using Context) {
   }
 
   private def genFixedSizeArray(st: SimpleType): nir.Type = {
+    def parseDigit(st: SimpleType): Int = {
+      try defnNir.NatBaseClasses.indexOf(st.sym)
+      catch {
+        case e: TypeError =>
+          // Can happen when Nat class is not yet availble, etc. usages withing nativelib
+          st.sym.name.toSimpleName.toString match
+            case s"Nat$$_${digit}" if digit.length == 1 =>
+              digit.toIntOption.getOrElse(throw e)
+            case _ => throw e
+      }
+    }
     def natClassToInt(st: SimpleType): Int =
-      if (st.targs.isEmpty) defnNir.NatBaseClasses.indexOf(st.sym)
+      if (st.targs.isEmpty) parseDigit(st)
       else
         st.targs.foldLeft(0) {
-          case (acc, st) => acc * 10 + defnNir.NatBaseClasses.indexOf(st.sym)
+          case (acc, st) => acc * 10 + parseDigit(st)
         }
 
     val SimpleType(_, Seq(elemType, size)) = st