[lld-macho] Simplify the handling of "no unwind info" functions

This diff does away with `addEntriesForFunctionsWithoutUnwindInfo()`,
because `addSymbol()` can now determine which functions need those
entries.

While overhauling UnwindInfoSection, I also parallelized the relocation
of the contents of the CUEs. This somewhat offsets the time regression
from creating one InputSection per CUE (which was done in D109944).

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D109945