[lld-macho][nfc] Track # of ICF calls to `equals*` methods

This is debug code that is disabled by default. It'll provide a easy way
to figure out the impact (if any) of tweaking ICF's hashing algorithm
(since a poor quality hash will result in many more `equals*` calls).

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D121051