[libcxx] Fix XFAILs for aligned allocation tests on older OSX versions

Summary:
Since r338934, Clang emits an error when aligned allocation functions are
used in conjunction with a system libc++ dylib that does not support those
functions. This causes some tests to fail when testing against older libc++
dylibs. This commit marks those tests as UNSUPPORTED, and also documents the
various reasons for the tests being unsupported.

Reviewers: vsapsai, EricWF

Subscribers: christof, dexonsmith, cfe-commits, mclow.lists, EricWF

Differential Revision: https://reviews.llvm.org/D50341

llvm-svn: 339743