[clang] Move CLANG_BOOTSTRAP_CMAKE_ARGS above PASSTHROUGH_VARIABLES

I want to pass some CMake cache files in CLANG_BOOTSTRAP_CMAKE_ARGS as
`-C <cache file>.cmake` arguments. I want to be able to use the values
of the bootstrap passthrough variables in the cache files, so the cache
file arguments need to be after passthrough variables. This should be
safe because the values of passthrough variables are all constants and
can't refer to values in CLANG_BOOTSTRAP_CMAKE_ARGS.

Differential Revision: https://reviews.llvm.org/D71428