Remove obsolete msvc workaround from AddLLVM.cmake

The original change to add the workaround is from 10 years ago and a lot has happened with msvc and cmake and llvm's usage of cmake since and we no longer need the workaround for any scenarios that I am aware of. Build more is now correctly configured for multi-configuration generators such as Visual Studio.

The workaround is, however, causing issues with some of the recent mlir tests as because of the workaround we cannot correctly determine whether assertions are enabled (see https://reviews.llvm.org/D105961).

The original change is:

```
commit b46fdac4
Author: Andrew Trick <atrick@apple.com>
Date:   Tue Jun 28 16:32:01 2011

    cmake: Our MSVC build does not support config-time build mode.

    llvm-svn: 134008
```

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D109521