Use strong enums independently of -fobjc-arc

r145698 introduced _LIBCPP_HAS_NO_STRONG_ENUMS by copy-pasting the
__has_feature check from objc_arc_weak/_LIBCPP_HAS_OBJC_ARC_WEAK, and
accidentally started defining _LIBCPP_HAS_NO_STRONG_ENUMS whenever
__has_feature(objc_arc_weak).  This is totally bogus, and means that
Libc++ thinks Objective-C++ compilations with -fobjc-arc don't have
strong enums.

Delete the accidental line.

I thought about adding a test, but it would be entirely duplicative of
the patch (if has-feature strong enums, check that has-no-strong-enums
is not defined).

llvm-svn: 299236