[driver][darwin] Don't use -platform_version flag by default (PR44813)

The code in llvmorg-10-init-12188-g25ce33a6 is a breaking change for
users of older linkers who don't pass a version parameter, which
prevents a drop-in clang upgrade. Old tools can't know about what future
tools will do, so as a general principle the burden should be new tools
to be compatible by default. Also, for comparison, none of the other
tests of Version within AddLinkArgs add any new behaviors unless the
version is explicitly specified. Therefore, this patch changes the
-platform_version behavior from opt-out to opt-in.

Patch by David Major!

Differential revision: https://reviews.llvm.org/D74784