Bump to xamarin/NRefactory@495405a6 (dotnet#5606)

Changes: xamarin/NRefactory@0607a4a...495405a

  * xamarin/NRefactory@495405a6: Merge pull request #2 from jpobst/sdk-style
  * xamarin/NRefactory@e4d5997e: Update some project files to SDK-style.
  * xamarin/NRefactory@60ac8e99: Merge pull request #1 from xamarin/cecilPackageReference
  * xamarin/NRefactory@c2c07caa: Switch Cecil reference to PackageReference.

We are working on using `dotnet build` to build `Xamarin.Android.sln`
and `Xamarin.Android-Tests.sln`.  `dotnet build` requires that all
projects be SDK-style short-form projects.

The `tests/MSBuildDeviceIntegration` integration tests use NRefactory
as part of the debugger tests, to ensure that the debugger can attach
to an app on-device.  In order for `tests/MSBuildDeviceIntegration`
to be buildable with `dotnet build`, NRefactory must have SDK-style
`.csproj` files.

Unfortunately, the [icsharpcode/NRefactory][0] repo only contains
"long-form" projects, has been archived, and is now read-only.

Switch to the [xamarin/NRefactory][1] repo, which is *not* archived
and is maintained by the VSMac team, and bump to a commit which
includes xamarin/NRefactory@e4d5997e, which migrates many of the
`.csproj` files to SDK-style short-form projects.

This allows us to continue our `dotnet build` explorations.

[0]: https://github.com/icsharpcode/NRefactory
[1]: https://github.com/xamarin/NRefactory

.gitmodules

@@ -24,7 +24,7 @@
     branch = master
 [submodule "external/nrefactory"]
     path = external/nrefactory
-    url = https://github.com/icsharpcode/NRefactory.git
+    url = https://github.com/xamarin/NRefactory.git
     branch = master
 [submodule "external/opentk"]
     path = external/opentk