[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.

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