[llvm-branch-commits] [Clang] Make -falloc-token-mode a hidden frontend option (PR #169359)

[Marco Elver via llvm-branch-commits]

https://github.com/melver created https://github.com/llvm/llvm-project/pull/169359

In preparation of moving AllocToken instrumentation to backend LTO
phases, we need to make this option a frontend option that can be
converted to the appropriate linker plugin command line option.

This removes the need to use `-Xclang` when setting the mode; document
it and update the existing tests.