Unescape Intrinsic Property Function Does Double-Unescaping

By calling the `Unescape` intrinsic property function (as in `$([MSBuild]::Unescape(...))`), I expect it to do one level of unescaping.

But I'm getting double unescaping instead. So turns out I have to do `Escape` twice on the data I'm feeding into the function to get the raw string back. This is not intuitive.

I guess this happens because one level of unescaping is done in ``Microsoft.Build.Evaluation.Expander`2+Function`1::Execute`` when preparing the argument (`args[n] = EscapingUtilities.UnescapeAll(argumentValue);`), and the second level of unescaping happens when the `Unescape` intrinsic function itself gets actually executed. Seems like it should have been a no-op function.

### Steps to reproduce

Suppose we're trying to load a task from a DLL in the folder named `$(The%20Folder)` (literally, with these `$` and `%` characters on disk, why not). You do not need the task DLL, just consider the path and match to the error messages when the task DLL is not found -- we need to get the desired path in the error message.

I've chosen `UsingTask` for this example because it skips unescaping on its path argument and thus avoids an extra level of possible unescaping.

Run the project file:
```xml

```

### Expected behavior

`... task could not be loaded from the assembly C:\$(The%20Folder)\Task.dll …` if the first `UsingTask` unescapes only one level of escaping.

### Actual behavior

`... task could not be loaded from the assembly C:\$(The Folder)\Task.dll …`, as two levels of escaping have been lifted.

If the first `UsingTask` element is commented out to let the second one work (that's doubly-escaped), we see the expected result.

### Environment data
`msbuild /version` output:
```
Microsoft (R) Build Engine version 15.8.169+g1ccb72aefa for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.8.169.51996
```
OS info: `Microsoft Windows [Version 10.0.17134.523]`

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):

PS What would be the proper workaround, safest around different MSBuild versions out there starting with v4.0, to emit a project file which would pass the task path correctly?