[clang] Upstream visionOS Availability & DarwinSDKInfo APIs (#84279)

Admittedly a bit awkward, `visionos` is the correct and accepted
spelling for annotating availability for xrOS target triples. This patch
detects errors and handles cases when `xros` is mistakenly passed.
In addition, add APIs for introduced/deprecated/obsoleted versioning in
DarwinSDKInfo mappings.