[cmake, unittests] Fix the CMake file for the LLDB unittests to support multiple configurations

Summary: The current setup for the unit tests only works correctly when the generator does not support multiple configurations. When the generator supports multiple configurations, the inputs are not copied to the correct per-configuration directory. This change sets up the build to copy the inputs in each configuration directory.

Reviewers: labath, asmith, zturner

Reviewed By: labath

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46642

llvm-svn: 331914