[Java.Interop.Tools.JavaCallableWrappers] Refactor (#1174)

Context: #540

Refactor `Java.Interop.Tools.JavaCallableWrappers` to be more
maintainable and modular.  Adopts the same architecture as
`generator`: using a `CecilImporter` to import data from a compiled
assembly into an intermediate model.  Java code is then generated
from the intermediate model.

Refactoring and updating this code should make it not only more
maintainable, but also easier to extend with potential upcoming
features we are considering:

  - Moving `jcw-gen` into the `ILLink` pipeline
  - [Sanity-check Java Callable Wrappers][0]
  - Possibly creating a Kotlin backend

[0]: dotnet/android#4278

Author: jpobst

src/Java.Interop.Tools.Diagnostics/Java.Interop.Tools.Diagnostics/Diagnostic.cs

@@ -3,7 +3,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-
+using System.Diagnostics.CodeAnalysis;
 using Mono.Cecil.Cil;
 
 namespace Java.Interop.Tools.Diagnostics {
@@ -148,6 +148,7 @@ namespace Java.Interop.Tools.Diagnostics {
 	//
 
 	public static class Diagnostic {
+		[DoesNotReturn]
 		public static void Error (int code, SequencePoint? location, string message, params object[] args)
 		{
 			throw new XamarinAndroidException (code, message, args) {
@@ -156,11 +157,13 @@ public static void Error (int code, SequencePoint? location, string message, par
 		}
 
 
+		[DoesNotReturn]
 		public static void Error (int code, string message, params object[] args)
 		{
 			throw new XamarinAndroidException (code, message, args);
 		}
 
+		[DoesNotReturn]
 		public static void Error (int code, Exception innerException, string message, params object[] args)
 		{
 			throw new XamarinAndroidException (code, innerException, message, args);