Preload handleRecursive to prevent nested stackoverflows

Thanks `@smarter` for guidance.

Author: Blaisorblade

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

@@ -75,6 +75,11 @@ class RecursionOverflow(val op: String, details: => String, previous: Throwable,
   override def getStackTrace() = previous.getStackTrace()
 }
 
+/** Post-process exceptions that might result from StackOverflow to add
+  * tracing information while unwalking the stack.
+  */
+// Beware: Since this object is only used when handling a StackOverflow, this code
+// cannot consume significant amounts of stack.
 object handleRecursive {
   def apply(op: String, details: => String, exc: Throwable, weight: Int = 1): Nothing = exc match {
     case _: RecursionOverflow =>

compiler/src/dotty/tools/package.scala

@@ -5,6 +5,11 @@ package object tools {
   class sharable extends Annotation
   class unshared extends Annotation
 
+  // Ensure this object is already classloaded, since it's only actually used
+  // when handling stack overflows and every operation (including class loading)
+  // risks failing.
+  dotty.tools.dotc.core.handleRecursive
+
   val ListOfNil = Nil :: Nil
 
   /** True if two lists have the same length.  Since calling length on linear sequences