[lld][thinlto] Include -mllvm options in the thinlto cache key

[lld][thinlto] Include -mllvm options in the thinlto cache key

They may modify thinlto optimization.

This patch only extends support for `-mllvm`. There is another way to
pass llvm flags, `-plugin-opt`, but its processing is different and will
be provided in a subsequent patch.

Differential Revision: https://reviews.llvm.org/D134013