[Triple] support macOS 11 os version number

macOS goes to 11! This commit adds support for the new version number by ensuring
that existing version comparison routines, and the 'darwin' OS identifier
understands the new numbering scheme. It also adds a new utility method
'getCanonicalVersionForOS', which lets users translate some uses of
macOS 10.16 into macOS 11. This utility method will be used in upcoming
clang and swift commits.

Differential Revision: https://reviews.llvm.org/D82337