Refactor ObjectFile::GetSDKVersion

Summary: This patch modernizes the GetSDKVersion API and hopefully prevents problems such as the ones discovered in D61218.

Reviewers: aprantl, jasonmolenda, clayborg

Reviewed By: aprantl, clayborg

Subscribers: clayborg, labath, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61233

llvm-svn: 365090