[lld-macho][nfc] De-templatize UnwindInfoSection

[lld-macho][nfc] De-templatize UnwindInfoSection

Follow-on to {D123276}. Now that we work with an internal
representation of compact unwind entries, we no longer need to template
our UnwindInfoSectionImpl code based on the pointer size of the target
architecture.

I've still kept the split between `UnwindInfoSectionImpl` and
`UnwindInfoSection`. I'd introduced that split in order to do type
erasure, but I think it's still useful to have in order to keep
`UnwindInfoSection`'s definition in the header file clean.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D123277