[libcxx] [test] Guard __has_include usage with a macro

Summary: There's a macro scheme already being used for __has_feature etc. Use it for __has_include too, which makes MSVC happy (it doesn't support __has_include yet, and unguarded use explodes horribly).

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25251

llvm-svn: 283260