Improve error reporting when Crossgen2 crashes on Managed C++

Based on Manish's feedback from another PR I investigated options
to improve Crossgen2 diagnostics in the presence of Managed C++
that currently crashes the metadata reader in several places.
This is the barebones version of such support, centralizing detection
and reporting of the failures. Right now we don't have any pretty
way of reporting localized strings from this low-level code, I'll
welcome any suggestions how to polish the change in that direction.

Thanks

Tomas

src/coreclr/src/tools/Common/TypeSystem/Ecma/EcmaMethod.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Reflection;
@@ -448,17 +449,32 @@ public override bool IsPInvoke
             }
         }
 
+        public void CheckManagedCPlusPlusPInvoke()
+        {
+            Debug.Assert(IsPInvoke);
+            MethodDefinition methodDef = MetadataReader.GetMethodDefinition(_handle);
+            MethodImport import = methodDef.GetImport();
+            if (import.Module.IsNil || MetadataReader.GetModuleReference(import.Module).Name.IsNil)
+            {
+                // Managed C++ PInvoke into the same module
+                throw new NotImplementedException(ToString());
+            }
+        }
+
         public override PInvokeMetadata GetPInvokeMethodMetadata()
         {
             if (!IsPInvoke)
                 return default(PInvokeMetadata);
 
+            CheckManagedCPlusPlusPInvoke();
+
             MetadataReader metadataReader = MetadataReader;
             MethodDefinition methodDef = metadataReader.GetMethodDefinition(_handle);
             MethodImport import = methodDef.GetImport();
             string name = metadataReader.GetString(import.Name);
 
             ModuleReference moduleRef = metadataReader.GetModuleReference(import.Module);
+
             string moduleName = metadataReader.GetString(moduleRef.Name);
 
             MethodImportAttributes importAttributes = import.Attributes;

src/coreclr/src/tools/Common/TypeSystem/IL/EcmaMethodIL.cs

@@ -28,6 +28,10 @@ public sealed partial class EcmaMethodIL : MethodIL
 
         public static EcmaMethodIL Create(EcmaMethod method, bool clearInitLocals = false)
         {
+            if (method.IsPInvoke)
+            {
+                method.CheckManagedCPlusPlusPInvoke();
+            }
             var rva = method.MetadataReader.GetMethodDefinition(method.Handle).RelativeVirtualAddress;
             if (rva == 0)
                 return null;