[lld-macho] Remove duplicate minimum version info

At some point PlatformInfo's Target changed types to a type that also
has minimum deployment target info. This caused ambiguity if you tried
to get the target triple from the Target, as the actual minimum version
info was being stored separately. This bulk of this change is changing
the parsing of these values to support this.

Differential Revision: https://reviews.llvm.org/D145263