No engine work

* Use Microsoft.NET.Sdk.WindowsDesktop for XAML projects

* MethodSymbol and FieldSymbol equality fixes:
 - Defer the equality of method/field symbol to the substituted symbol when compared
 - Make the hashcode of substituted method/field defer to the original definition

Rename from IncludeNonNullableReferenceTypeModifier to IncludeNotNullableReferenceTypeModifier (#41332)

* Fix Pythia pooled objects wrappers

* Fix RPC

Our best guess is we ran naming styles on open files only for two
reasons:

1. At the time, we only supported .editorconfig in-proc, so
   out of proc wouldn't have used the right settings.
2. Even if we were running in-proc, there might have been performance
   issues since we were running across a lot of files.

Neither of these concerns apply at this point: we now support
.editorconfig out of proc (both in the legacy path and the new path) and
since we're already running other analyzers that use .editorconfig,
the cost of processing it will exist regardless.

Closes #41304

This API has always accepted null as a shortcut for an empty array, so
this annotates it accordingly.

Had to clean up a few nullable annotations now that we are compiling
agaist `netcoreapp3.1` and hence get the full value of the framework
annotations.

This is also problematic though because there are now two places where
the compiler can see nullable attributes that are directly used by the
developer. For example `NotNullWhenAttribute`. This is both defined in
our assemblies for non-netcoreapp target frameworks and provided by the
SDK when targeting `netcoreapp3.1`.

This causes a problem for assemblies which have the following
characteristics:

1. Target `netcoreapp3.1`
1. Reference an assembly targeting `netstandard2.0` which uses our
nullable attributes definition
1. Has IVT into (2) above

These properties essentially define all of our unit test assemblies. In
that environment it's not possible to use nullable attributes in code
because the compiler can't disambiguate which definition of
`NotNullWhenAttribute` to use. This meant I had to temporarily remove a
few attributes until we can complete #40766.