[lld-macho] Implement ObjC category merging (-objc_category_merging) (#85727)

This change adds a flag to lld to enable category merging for MachoO +
ObjC.
It adds the '-objc_category_merging' flag for enabling this option and
uses the existing '-no_objc_category_merging' flag for disabling it.
In ld64, this optimization is enabled by default, but in lld, for now,
we require explicitly passing the '-objc_category_merging' flag in order
to enable it.

Behavior: if in the same link unit, multiple categories are extending
the same class, then they get merged into a single category.
Ex: Cat1(method1+method2,protocol1) + Cat2(method3+method4,protocol2,
property1) = Cat1_2(method1+method2+method3+method4,
protocol1+protocol2, property1)

Notes on implementation decisions made in this diff:

There is a possibility to further improve the current implementation by
directly merging the category data into the base class (if the base
class is present in the link unit) - this improvement may be done a...