[CMake] Have check-lldb-unit use CMAKE_CURRENT_BINARY_DIR

llvm-lit uses `map_config` directives (generated at configuration-time) to
make it possible to pass a test path relative to the source instead of
the build folder (CMAKE_CURRENT_BINARY_DIR).

However, this doesn't work in the case of swift where the build
directory layout prevents llvm-lit from knowing about lldb and its test
paths. This caused check-lldb-unit to fail in this configuration, while
everything was working as expected upstream. This patch fixes the issue
by passing a path relative to the build directory. This was already the
case for check-lldb-lit.

llvm-svn: 337291