[generator] Fix MSBuild warning/error format for Visual Studio (#765)

In b858dc5 we updated `generator` warnings/errors to give line &
column information in more places.  However, the existing method for
formatting the line & column information was wrong:

	// Correct
	C:\code\Metadata.xml(2, 6): warning BG8A04: <attr path="/api/package[@name='androidx.appcompat.widget']/class[@name='RoundRectDrawableWithShadow']"/> matched no nodes.

	// Incorrect
	C:\code\Metadata.xml(2, 6) warning BG8A04: <attr path="/api/package[@name='androidx.appcompat.widget']/class[@name='RoundRectDrawableWithShadow']"/> matched no nodes.

By omitting the colon after the line & column information,
Visual Studio parses the colon within `C:\` instead, resulting in:

![image](https://user-images.githubusercontent.com/179295/102400384-7500b100-3fa7-11eb-8f35-aae2a04f3a58.png)

  * Filename: `C`
  * Line number: 1

This is actually worse than what we previously had, as double-clicking
it does nothing, as `C` is not a valid file on disk.

Rewrite the `Report.Format()` method to be a little clearer to read
and add the required colon.

tests/generator-Tests/Unit-Tests/ReportTests.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using MonoDroid.Generation;
+using NUnit.Framework;
+
+namespace generatortests
+{
+	public class ReportTests
+	{
+		[Test]
+		public void FormatTests ()
+		{
+			var code = 0x37;
+			var msg = "There was a {0} error";
+			var args = "bad";
+			var sourcefile = @"C:\code\test.cs";
+			var line = 32;
+			var col = 12;
+
+			Assert.AreEqual ("error BG0037: There was a bad error", Report.Format (true, code, null, 0, 0, msg, args));
+			Assert.AreEqual (@"C:\code\test.cs: error BG0037: There was a bad error", Report.Format (true, code, sourcefile, 0, 0, msg, args));
+			Assert.AreEqual (@"C:\code\test.cs(32): error BG0037: There was a bad error", Report.Format (true, code, sourcefile, line, 0, msg, args));
+			Assert.AreEqual (@"C:\code\test.cs(32, 12): error BG0037: There was a bad error", Report.Format (true, code, sourcefile, line, col, msg, args));
+			Assert.AreEqual (@"C:\code\test.cs(32, 12): warning BG0037: There was a bad error", Report.Format (false, code, sourcefile, line, col, msg, args));
+		}
+	}
+}

tools/generator/Utilities/Report.cs

@@ -129,9 +129,25 @@ public static string FormatCodedMessage (bool error, LocalizedMessage message, p
 
 		public static string Format (bool error, int errorCode, string sourceFile, int line, int column, string format, params object[] args)
 		{
-			var origin = sourceFile != null ? sourceFile + (line > 0 ? column > 0 ? $"({line}, {column})" : $"({line})" : null) + ' ' : null;
-			return string.Format ("{0}{1} BG{2:X04}: ", origin, error ? "error" : "warning", errorCode) +
-				string.Format (format, args);
+			var origin = FormatOrigin (sourceFile, line, column);
+
+			return $"{origin}{(error ? "error" : "warning")} BG{errorCode:X04}: " + string.Format (format, args);
+		}
+
+		static string FormatOrigin (string sourceFile, int line, int column)
+		{
+			if (string.IsNullOrWhiteSpace (sourceFile))
+				return null;
+
+			var ret = sourceFile;
+
+			if (line == 0)
+				return ret + ": ";
+
+			if (column > 0)
+				return ret + $"({line}, {column}): ";
+
+			return ret + $"({line}): ";
 		}
 	}