Set EmbedUntrackedSources to true by default (#4063)

[tmat]

Description

Port #4063 to 3.x

Customer Impact

Enables embedding generated source files to PDB by default. Required for Source Link validation.

Regression

No

Risk

Small.

Workarounds

None.

[JohnTortugo]

AFAIU chances of this breaking someone is very very low. Right?!

[tmat]

Yes, break seems unlikely. If it does that would be a product bug and we would definitely want to know about that.

[wtgodbe]

@tmat we think this caused a break in WPF: dotnet/wpf#3272

After this change, PresentationUI fails to build:

artifacts\obj\PresentationUI\Debug\netcoreapp3.1\InstallationError.g.cs#L40
artifacts\obj\PresentationUI\Debug\netcoreapp3.1\InstallationError.g.cs(40,18): error CS1504: Source file 'InstallationError.xaml' could not be opened -- Could not find file.

These 'embed' parameters were not passed to csc before the update:

/embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\InstallationError.g.cs /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\TenFeetInstallationError.g.cs /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\TenFeetInstallationProgress.g.cs /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\MS\Internal\Documents\FindToolbar.g.cs /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\MS.Internal.Documents.Resources.cs /embed:"D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1.NETCoreApp,Version=v3.1.AssemblyAttributes.cs" /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\System.Windows.TrustUI.SRID.cs /embed:D:\wpf2\artifacts\obj\PresentationUI\Debug\netcoreapp3.1\PresentationUI_rdq3st4q_wpftmp.AssemblyInfo.c