[lit] Generate a single lit cfg file for tests that require dotest.py

The current way that the lit configuration is generated for the LLDB
tests that run using dotest causes cmake to fail when using a generator
which supports multiple configurations (such as Visual Studio). The
failure is because file GENERATE will create a file *per possible
configuration* resulting in the same lit configuration file being
overwritten multiple times.

To fix the issue, we need to create a single lit file that is agnostic
of the configurations and can be used for any configuration.

Patch by: Stella Stamenova

Differential revision: https://reviews.llvm.org/D45918

llvm-svn: 330518