[Driver] Multilib YAML parsing

The format includes a ClangMinimumVersion entry to avoid a potential
source of subtle errors if an older version of Clang were to be used
with a multilib.yaml that requires a newer Clang to work correctly.
This feature is comparable to CMake's cmake_minimum_required.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D142932