Add ability to debug misfiring SuspendException

It should never reach the user, but if it does because of a bug like in #18517,
we should have a flag to find out where it came from.

compiler/src/dotty/tools/dotc/CompilationUnit.scala

@@ -132,7 +132,7 @@ class CompilationUnit protected (val source: SourceFile, val info: CompilationUn
 
 object CompilationUnit {
 
-  class SuspendException extends Exception with NoStackTrace
+  class SuspendException extends Exception with StackTraceOnDebug
 
   /** Make a compilation unit for top class `clsd` with the contents of the `unpickled` tree */
   def apply(clsd: ClassDenotation, unpickled: Tree, forceTrees: Boolean)(using Context): CompilationUnit =

compiler/src/dotty/tools/dotc/core/TypeErrors.scala

@@ -18,8 +18,7 @@ import collection.mutable
 
 import scala.annotation.constructorOnly
 
-abstract class TypeError(using creationContext: Context) extends Exception(""):
-
+trait StackTraceOnDebug extends Throwable:
   /** Will the stack trace of this exception be filled in?
    *  This is expensive and only useful for debugging purposes.
    */
@@ -35,6 +34,8 @@ abstract class TypeError(using creationContext: Context) extends Exception(""):
     if computeStackTrace then super.fillInStackTrace().nn
     else this
 
+abstract class TypeError(using creationContext: Context) extends Exception("") with StackTraceOnDebug:
+
   /** Convert to message. This takes an additional Context, so that we
    *  use the context when the message is first produced, i.e. when the TypeError
    *  is handled. This makes a difference for CyclicErrors since we need to know