[libc++] Remove redundant conditionals for Apple platforms

Summary:
In a bunch of places, we used to check whether LIBCXX_BUILDING_LIBCXXABI
is defined OR we're building for an Apple platform. This used to
be necessary in a time when Apple's build script did NOT define
LIBCXX_BUILDING_LIBCXXABI. However this is not relevant anymore
since Apple's build does define LIBCXX_BUILDING_LIBCXXABI.

Reviewers: EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60842

llvm-svn: 358988