[CMake] Have check-lldb-unit use CMAKE_CURRENT_BINARY_DIR
[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.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@337291 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit e71634d556cf23871b427126d15adf9fc9c05283)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb