Do not require __STDC_LIMIT_MACROS and others

Summary: Do not require __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS macros to be defined globally. They are not needed for C++11 compliant standard headers.

Reviewers: joerg, jyknight

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21553

llvm-svn: 273493