XFAIL: libcpp-no-deduction-guides in...

XFAIL: libcpp-no-deduction-guides in libcxx/test/std/strings/basic.string/string.cons/iter_alloc_deduction.fail.cpp

Summary: Refactor the previous version method of marking each apple-clang version as UNSUPPORTED and just XFAIL'ing the libcpp-no-deduction-guides instead.  This brings this test inline with the same style as iter_alloc_deduction.pass.cpp

Reviewers: EricWF, dexonsmith

Reviewed By: EricWF

Subscribers: EricWF, vsapsai, vsk, cfe-commits

Differential Revision: https://reviews.llvm.org/D44103

llvm-svn: 327178