Add .clang-format to libc++

Adding a Clang Format file to libc++ and which style it should use has been
discussed a couple of times. This patch finally adds a .clang-format file
which specifies LLVM styles.

Personally I dislike how the LLVM style handles much of the meta-programming
in libc++. However the general consensus was that libc++ should prefer the
LLVM style and make adjustments where needed.

Note that using clang-format on changes is not required, especially for
changes within the headers. However formatting tests should be encouraged.

llvm-svn: 287020