Remove libc++ checks and workarounds for unsupported old versions of GCC (<4.9).

Differential Revision: https://reviews.llvm.org/D61107


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359232 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed