[llvm][TextAPI] Handle implicitly upgraded deployment versions

Sometimes the clang driver will receive a target triple where the
deployment version is too low to support the platform + arch. In those
cases, the compiler upgrades the final minOS which is what gets recorded
ultimately by the linker in LC_BUILD_VERSION. TextAPI should also reuse
this logic for capturing minOS in recorded TBDv5 files.

Reviewed By: ributzka

Differential Revision: https://reviews.llvm.org/D145690