[CodeComplete] Default parameter names of completed closure to internal names

If have a function that takes a trailing closure as follows
```
func sort(callback: (_ left: Int, _ right: Int) -> Bool) {}
```
completing a call to `sort` and expanding the trailing closure results in
```
sort { <#Int#>, <#Int#> in
  <#code#>
}
```

We should be doing a better job here and defaulting the trailing closure's to the internal names specified in the function signature. I.e. the final result should be
```
sort { left, right in
  <#code#>
}
```

This commit does exactly that.

Firstly, it keeps track of the closure's internal names (as specified in the declaration of `sort`) in the closure's type through a new `InternalLabel` property in `AnyFunctionType::Param`. Once the type containing the parameter gets canonicalized, the internal label is dropped.

Secondly, it adds a new option to `ASTPrinter` to always try and print parameter labels. With this option set to true, it will always print external paramter labels and, if they are present, print the internal parameter label as `_ <internalLabel>`.

Finally, we can use this new printing mode to print the trailing closure’s type as
```
<#T##callback: (Int, Int) -> Bool##(_ left: Int, _ right: Int) -> Bool#>
```

This is already correctly expanded by code-expand to the desired result. I also added a test case for that behaviour.

lib/IDE/CodeCompletion.cpp

@@ -1001,6 +1001,7 @@ void CodeCompletionResultBuilder::addCallParameter(Identifier Name,
     PO.SkipAttributes = true;
     PO.OpaqueReturnTypePrinting =
         PrintOptions::OpaqueReturnTypePrintingMode::WithoutOpaqueKeyword;
+    PO.AlwaysTryPrintParameterLabels = true;
     if (ContextTy)
       PO.setBaseType(ContextTy);
 
@@ -1017,6 +1018,8 @@ void CodeCompletionResultBuilder::addCallParameter(Identifier Name,
 
         if (param.hasLabel()) {
           OS << param.getLabel();
+        } else if (param.hasInternalLabel()) {
+          OS << param.getInternalLabel();
         } else {
           OS << "<#";
           if (param.isInOut())
@@ -2337,8 +2340,7 @@ public:
       SmallVector<AnyFunctionType::Param, 8> erasedParams;
       for (const auto &param : genericFuncType->getParams()) {
         auto erasedTy = eraseArchetypes(param.getPlainType(), genericSig);
-        erasedParams.emplace_back(erasedTy, param.getLabel(),
-                                  param.getParameterFlags());
+        erasedParams.emplace_back(param.withType(erasedTy));
       }
       return GenericFunctionType::get(genericSig,
           erasedParams,