Merge branch 'Ensure type tags are always ordered first in BTF'

Kumar Kartikeya Dwivedi says:

====================

When iterating over modifiers, ensure that type tags can only occur at head of
the chain, and don't occur later, such that checking for them once in the start
tells us there are no more type tags in later modifiers. Clang already ensures
to emit such BTF, but user can craft their own BTF which violates such
assumptions if relied upon in the kernel.

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20220418224719.1604889-1-memxor@gmail.com

 * Address nit from Yonghong, add Acked-by

v1 -> v2
v1: https://lore.kernel.org/bpf/20220406004121.282699-1-memxor@gmail.com

 * Fix for bug pointed out by Yonghong
 * Update selftests to include Yonghong's example
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>