[Sema] Improve diagnostic message for unavailable C++17 aligned

allocation functions.

This changes the error message Sema prints when an unavailable C++17
aligned allocation function is selected.

Original message: "... possibly unavailable on x86_64-apple-macos10.12"
New message: "... only available on macOS 10.13 or newer"

This is a follow-up to r306722.

rdar://problem/32664169

Differential Revision: https://reviews.llvm.org/D35520

llvm-svn: 308496