AIX system headers need stdint.h and inttypes.h to be re-enterable

Summary:
AIX system headers need stdint.h and inttypes.h to be re-enterable when macro _STD_TYPES_T is defined so that limit macro definitions such as UINT32_MAX can be found. This patch attempts to allow that on AIX.

Reviewers: hubert.reinterpretcast, jasonliu, mclow.lists, EricWF

Reviewed by: hubert.reinterpretcast, mclow.lists

Subscribers: jfb, jsji, christof, cfe-commits, libcxx-commits, llvm-commits

Tags: #LLVM, #clang, #libc++

Differential Revision: https://reviews.llvm.org/D59253

llvm-svn: 363939