[llvm-reduce] Display all relevant options in -help

Previously the options category given to cl::HideUnrelatedOptions was
local to llvm-reduce.cpp and as a result only options declared in that
file were visible in the -help options listing. This was a bit
unfortunate since there were several useful options declared in other
files. This patch addresses that.

Differential Revision: https://reviews.llvm.org/D118682