Remove a broken attempt to cope with someone #undef'ing __has_include_next.
This was broken because __has_include_next(...) would not be valid in a
preprocessor condition if __has_include_next is not defined.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201731 91177308-0d34-0410-b5e6-96231b3b80d8