[CSGen] No longer allow _ allow to be a hole directly

Problems related to incorrect use of `_` where previously diagnosed
by syntactic diagnostics which meant that it could only happen on
type-checked AST. This is no longer a case so we don't have to allow
incorrect uses of `_` to form solution without any fixes.

Author: xedin

lib/Sema/CSGen.cpp

@@ -3068,8 +3068,7 @@ namespace {
       }
 
       auto locator = CS.getConstraintLocator(expr);
-      auto typeVar = CS.createTypeVariable(locator, TVO_CanBindToNoEscape |
-                                                    TVO_CanBindToHole);
+      auto typeVar = CS.createTypeVariable(locator, TVO_CanBindToNoEscape);
       return LValueType::get(typeVar);
     }