[TextAPI] Make min-os deployment version optional in tbd-v5.

The minOS version is recorded in tbd-v5 so the linker can report
diagnostics when a library and client are misconfigured.

Dylibs should always have a minOS recorded, but in was not recorded in
previous TBD versions. To accommodate the format transition, treat
unrecorded minOS versions as 0.

Reviewed By: zixuw

Differential Revision: https://reviews.llvm.org/D156487