[Sema] Teach Clang that aligned allocation is not supported with macosx10.13

Summary:
r306722 added diagnostics when aligned allocation is used with deployment
targets that do not support it, but the first macosx supporting aligned
allocation was incorrectly set to 10.13. In reality, the dylib shipped
with macosx10.13 does not support aligned allocation, but the dylib
shipped with macosx10.14 does.

Reviewers: ahatanak

Subscribers: christof, jkorous, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D56445

llvm-svn: 350649