[llvm-jitlink] Generalize statistics gathering / reporting.

Moves the llvm-jitlink tool statistics out of the Session struct and into a new
LLVMJITLinkStatistics class.

Also removes the `-show-sizes` option. Each statistic added will now have its
own option. The two previous stats (total size of all blocks before pruning and
after fixups) are now available as -pre-prune-total-block-size and
-post-fixup-total-block-size.

This change should make it easier to add new statistics.