[BPF][DebugInfo][NFC] Move BTF.h definitions from BPF target to DebugInfo

There are plans to add some BTF processing to tools like objdump and
readelf. This commit moves BTF.{h,def} files from BPF target specific
location to include/llvm/DebugInfo/* to avoid tools including headers
from lib/Target/*.

Reviewed By: yonghong-song, MaskRay

Differential Revision: https://reviews.llvm.org/D149501