Fall-back assignment of closure discriminators in ill-formed code

There are some cases of severely malformed code where we cannot compute
closure discriminators appropriate. Introduce a fallback to use the
unstable global numbering of discriminators in this case.

Author: DougGregor

lib/AST/Expr.cpp

@@ -1864,6 +1864,16 @@ unsigned AbstractClosureExpr::getDiscriminator() const {
 
   evaluateOrDefault(
       getASTContext().evaluator, LocalDiscriminatorsRequest{getParent()}, 0);
+
+  // Ill-formed code might not be able to assign discriminators, so assign
+  // a new one now.
+  if (getRawDiscriminator() == InvalidDiscriminator &&
+      getASTContext().Diags.hadAnyError()) {
+    const_cast<AbstractClosureExpr *>(this)->
+        Bits.AbstractClosureExpr.Discriminator =
+          getASTContext().NextAutoClosureDiscriminator++;
+  }
+
   assert(getRawDiscriminator() != InvalidDiscriminator);
   return getRawDiscriminator();
 }