Csc invoked in XamlPrecompile doesn't get Nullable passed

[huoyaoyuan]

Issue Description

Csc invoked in XamlPrecompile doesn't get Nullable passed, and results in warning for NRT disabled when the project file has <Nullable>enable</Nullable>.

Root cause of dotnet/roslyn#46420 .

Steps to Reproduce

1. Create a WinUI 3 template application.
2. Ensure there is some xaml.
3. Add <Nullable>enable</Nullable> in project file.
4. Write some code with nullable reference type, such as a string? field in App.

Expected Behavior

Build without warning.

Actual Behavior

The IDE reports no warning. Build output reports warning CS8632: nullable reference types can only be used under #nullable enable.

Analysis

https://github.com/dotnet/roslyn/blob/4f1711c578fd260221f968696ab07b125581b9e1/src/Compilers/Core/MSBuildTask/Microsoft.CSharp.Core.targets#L113

The <Nullable> property is missing in Microsoft.CSharp.CurrentVersion.targets:

msbuild/src/Tasks/Microsoft.CSharp.CurrentVersion.targets

Lines 277 to 281 in ec6ed83

	NoConfig="true"
	NoLogo="$(NoLogo)"
	NoStandardLib="$(NoCompilerStandardLib)"
	NoWin32Manifest="$(NoWin32Manifest)"
	Optimize="$(Optimize)"

Versions & Configurations

Visual Studio 16.8 - 16.9 Preview 3

Attach a binlog

(Not necessary I think)