Resx code file not generated when referenced in WPF project

* .NET Core Version: 3.1.102
* Windows version: Windows 10 1909
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No
* Security issues and bugs should be reported privately, learn more via our [responsible disclosure guidelines](https://github.com/dotnet/wpf/blob/master/README.md#reporting-security-issues-and-security-bugs).

**Problem description:**
ResX code file does not get generated for WPF project as soon as the file is referenced (either from xaml or cs).

When all references are removed the file gets generated.

Works for WinForms and Console applications.

**Actual behavior:**
File not generated leading to compile error of "Cannot find the type..."

**Expected behavior:**
File generated and resolved by the rest of the application.

**Minimal repro:**
[reswpf.zip](https://github.com/dotnet/wpf/files/4346184/reswpf.zip)
[resnowpf.zip](https://github.com/dotnet/wpf/files/4346199/resnowpf.zip)