[lld-macho] Include ICF safe thunks in balanced partitioning (#212096)

Mach-O balanced partitioning currently discovers candidate sections by
walking the input-file section graph. In `--icf=safe_thunks` mode,
address-significant functions can instead be emitted as linker-created
thunk sections after that graph has been built. A temporal-profile name
then resolves to the dead folded input while the callable section
present in the final binary is never eligible for BP ordering.

Factor candidate collection into a helper, then inspect the final
`inputSections` set for sections containing a `Defined` symbol marked
`ICFFoldKind::Thunk`. This makes emitted ICF safe thunks visible while
keeping unrelated synthetic metadata outside BP.

The new arm64 regression folds a profiled address-significant function
into a 4-byte safe thunk and verifies both the BP startup count and
final symbol order. Without the change, BP orders zero startup sections
for that profile; with the change it orders the emitted thunk first.

GitOrigin-RevId: 25d130fc7efffd4d6b87ad9ab5193ce10cf37edb