Delay initialization of OptBisect

When LLVM is used in other projects, it may happen that global cons-
tructors will execute before the call to ParseCommandLineOptions.
Since OptBisect is initialized via a constructor, and has no ability
to be updated at a later time, passing "-opt-bisect-limit" to the
parse function may have no effect.

To avoid this problem use a cl::cb (callback) to set the bisection
limit when the option is actually processed.

Differential Revision: https://reviews.llvm.org/D104551