[generator] Fix a potential compile time error in generated *Async methods (#1536)

This can happen when a parameter of the method being [Async]-ified has
the same name as one of the delegate parameter.

E.g.

* Binding code

```
	delegate void MTKTextureLoaderCallback ([NullAllowed] IMTLTexture texture, [NullAllowed] NSError error);

	[Async]
	void FromTexture (MDLTexture texture, [NullAllowed] MTKTextureLoaderOptions options, MTKTextureLoaderCallback completionHandler);
```

* Generated code

```
		public unsafe virtual Task<global::Metal.IMTLTexture> FromTextureAsync (global::ModelIO.MDLTexture texture, NSDictionary options)
		{
			var tcs = new TaskCompletionSource<global::Metal.IMTLTexture> ();
>			FromTexture(texture, options, (texture, error) => {
				if (error != null)
					tcs.SetException (new NSErrorException(error));
				else
					tcs.SetResult (texture);
			});
			return tcs.Task;
		}
```

* Errpr

```
MTKTextureLoader.g.cs(397,35): error CS0136: A local variable named `texture' cannot be declared in this scope because it would give a different meaning to `texture', which is already used in a `parent or current' scope to denote something else
```

Note: we are appending, not prepending, a '_' character

Because:

* we don't want to pick up a fight with GetUniqueParamName that already
  prepend a '_' in some cases;

* Pre-fixing '_' does not work for parameters names starting with a `@`

src/generator.cs

@@ -4287,15 +4287,18 @@ public partial class Generator : IMemberGatherer {
 		throw new BindingException (1023, true, "Async method {0} with more than one result parameter in the callback by neither ResultTypeName or ResultType", minfo.mi);
 	}
 
-	string GetInvokeParamList (ParameterInfo[] parameters)
+	string GetInvokeParamList (ParameterInfo [] parameters, bool suffix = true)
 	{
 		StringBuilder sb = new StringBuilder ();
 		bool comma = false;
 		foreach (var pi in parameters) {
-			if (comma)
+			if (comma) {
 				sb.Append (", ");
+			}
 			comma = true;
 			sb.Append (pi.Name.GetSafeParamName ());
+			if (suffix)
+				sb.Append ('_');
 		}
 		return sb.ToString ();
 	}
@@ -4360,7 +4363,7 @@ public partial class Generator : IMemberGatherer {
 		print ("var tcs = new TaskCompletionSource<{0}> ();", ttype);
 		print ("{6}{5}{4}{0}({1}{2}({3}) => {{",
 		       mi.Name,
-		       GetInvokeParamList (minfo.async_initial_params),
+		       GetInvokeParamList (minfo.async_initial_params, false),
 		       minfo.async_initial_params.Length > 0 ? ", " : "",
 		       GetInvokeParamList (minfo.async_completion_params),
 		       minfo.is_extension_method ? "This." : string.Empty,
@@ -4372,8 +4375,8 @@ public partial class Generator : IMemberGatherer {
 		int nesting_level = 1;
 		if (minfo.has_nserror && !tuple) {
 			var var_name = minfo.async_completion_params.Last ().Name.GetSafeParamName ();;
-			print ("if ({0} != null)", var_name);
-			print ("\ttcs.SetException (new NSErrorException({0}));", var_name);
+			print ("if ({0}_ != null)", var_name);
+			print ("\ttcs.SetException (new NSErrorException({0}_));", var_name);
 			print ("else");
 			++nesting_level; ++indent;
 		}
@@ -4383,9 +4386,9 @@ public partial class Generator : IMemberGatherer {
 		else if (tuple) {
 			var cond_name = minfo.async_completion_params [0].Name;
 			var var_name = minfo.async_completion_params.Last ().Name;
-			print ("tcs.SetResult (new Tuple<bool,NSError> ({0}, {1}));", cond_name, var_name);
+			print ("tcs.SetResult (new Tuple<bool,NSError> ({0}_, {1}_));", cond_name, var_name);
 		} else if (minfo.is_single_arg_async)
-			print ("tcs.SetResult ({0});", minfo.async_completion_params [0].Name);
+			print ("tcs.SetResult ({0}_);", minfo.async_completion_params [0].Name);
 		else
 			print ("tcs.SetResult (new {0} ({1}));",
 				GetAsyncTaskType (minfo),

src/metalkit.cs

@@ -291,10 +291,12 @@ namespace XamCore.MetalKit {
 
 		[iOS (10,0)][Mac (10,12, onlyOn64 : true)]
 		[Export ("newTextureWithMDLTexture:options:completionHandler:")]
+		[Async]
 		void FromTexture (MDLTexture texture, [NullAllowed] NSDictionary options, MTKTextureLoaderCallback completionHandler);
 
 		[iOS (10,0)][Mac (10,12, onlyOn64 : true)]
 		[Wrap ("FromTexture (texture, options == null ? null : options.Dictionary, completionHandler)")]
+		[Async]
 		void FromTexture (MDLTexture texture, [NullAllowed] MTKTextureLoaderOptions options, MTKTextureLoaderCallback completionHandler);
 
 		[iOS (10,0)][Mac (10,12, onlyOn64 : true)]