[clang] Aligned allocation is actually supported in macosx 10.13

Summary:
In r350649, I changed aligned allocation from being available starting
in macosx10.13 to macosx10.14. However, aligned allocation is indeed
available starting with macosx10.13, my investigation had been based
on the wrong libc++abi dylib.

This means that Clang before the fix will be more stringent when it
comes to aligned allocation -- it will not allow it when back-deploying
to macosx 10.13, when it would actually be safe to do so.

Note that a companion change will be coming to fix the libc++ tests.

Reviewers: ahatanak

Subscribers: jkorous, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60626

llvm-svn: 358409