[libcxx] use clang's __char16_t and __char32_t definitions on Linux in C++03 mode.

Summary: This fixes ODR violations in C++03 mode in test/localization/locale.stdcvt. The special case for linux was introduced in 2010 before clang always defined __char16_t and __char32_t.

Reviewers: mclow.lists, danalbert, jroelofs, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5930