[libc++] Remove _LIBCPP_BUILDING_XXX macros, which are redundant since _LIBCPP_BUILDING_LIBRARY

Summary: As suggested by Marshall in https://reviews.llvm.org/D49914

Reviewers: mclow.lists, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50008

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338475 91177308-0d34-0410-b5e6-96231b3b80d8