[CodeCompletion] Don't report an extra argument failure if the code completion token is the first one in a call

This makes sure we don't penalize overloads with no arguments when calling
```swift
func foo() {}
func foo(_ arg: Int) {}
foo(#^COMPLETE^#
```

Author: ahoppen

lib/Sema/CSSimplify.cpp

@@ -1275,6 +1275,9 @@ class CompletionArgumentTracker : public ArgumentFailureTracker {
     if (ArgInfo.isBefore(argIdx)) {
       return false;
     }
+    if (argIdx == 0 && ArgInfo.completionIdx == 0) {
+      return false;
+    }
     return ArgumentFailureTracker::extraArgument(argIdx);
   }