Log Message:
lang: Import libcxx version 6.0.1.

libc++ is a new implementation of the C++ standard library, targeting
C++11.

Features and Goals
 * Correctness as defined by the C++11 standard.
 * Fast execution.
 * Minimal memory use.
 * Fast compile times.
 * ABI compatibility with gcc's libstdc++ for some low-level features
   such as exception objects, rtti and memory allocation.
 * Extensive unit tests.