AST: Make absence of discriminator into a fatal error

... even in noassert builds. This is always a miscompile and should
be prevented.

Author: slavapestov

lib/AST/Expr.cpp

@@ -1934,7 +1934,12 @@ unsigned AbstractClosureExpr::getDiscriminator() const {
         Bits.AbstractClosureExpr.Discriminator = discriminator;
   }
 
-  assert(getRawDiscriminator() != InvalidDiscriminator);
+  if (getRawDiscriminator() == InvalidDiscriminator) {
+    llvm::errs() << "Closure does not have an assigned discriminator:\n";
+    dump(llvm::errs());
+    abort();
+  }
+
   return getRawDiscriminator();
 }