Try to fix broken build due to r336663.

It turns out that the `${XRAY_HEADERS}` CMake variable was already
in use and was used for public headers. It seems that
`lib/xray/tests/CMakeLists.txt` was depending on this.

To fix rename the new `${XRAY_HEADERS}` to `${XRAY_IMPL_HEADERS}`.

llvm-svn: 336699