Have BCMarker::operator== take a const BCMarker&.

Summary: It was taking a BCMarker, which wreaked havoc with gdb. Running
a.BCMarker::operator==(b) was returning false, even when
memcmp(&a, &b, sizeof(BCMarker)) was returning 0 (all in gdb).

Reviewed By: @bertmaher

Differential Revision: D1636559