[CS] Add conversion for ErrorExpr's original expr

Make sure we don't produce unnecessary diagnostics while still allowing
things like cursor info to work. No test change since it's covered by
the next commit.

Author: hamishknight

lib/Sema/CSGen.cpp

@@ -1322,7 +1322,18 @@ namespace {
     }
 
     virtual Type visitErrorExpr(ErrorExpr *E) {
-      return recordInvalidNode(E);
+      auto ty = recordInvalidNode(E);
+      // If we have an original expression, introduce a conversion to the hole
+      // type of the ErrorExpr. This avoids unnecessary diagnostics for the
+      // original expression in cases where the ErrorExpr could have provided
+      // contextual info, while also still allowing the original expression to
+      // be solved without holes being introduced prematurely, allowing e.g
+      // cursor info to work correctly.
+      if (auto *orig = E->getOriginalExpr()) {
+        CS.addConstraint(ConstraintKind::Conversion, CS.getType(orig), ty,
+                         CS.getConstraintLocator(E));
+      }
+      return ty;
     }
 
     virtual Type visitCodeCompletionExpr(CodeCompletionExpr *E) {

lib/Sema/CSSimplify.cpp

@@ -5582,6 +5582,10 @@ bool ConstraintSystem::repairFailures(
     if (!anchor)
       return false;
 
+    // If we have an ErrorExpr anchor, we don't need to do any more fixes.
+    if (isExpr<ErrorExpr>(anchor))
+      return true;
+
     // This could be:
     // - `InOutExpr` used with r-value e.g. `foo(&x)` where `x` is a `let`.
     // - `ForceValueExpr` e.g. `foo.bar! = 42` where `bar` or `foo` are